Revert "Removed unused includes"

This reverts commit c9eb8f82e9febeb634a23bec6aeea915eb25fe26.

The change caused "implicit declaration" warning and actual segfault.

```
/tmp/ruby/v2/src/trunk-gc-asserts/enc/gb2312.c: In function ‘Init_gb2312’:
/tmp/ruby/v2/src/trunk-gc-asserts/enc/gb2312.c:6:31: warning: implicit declaration of function ‘rb_enc_find’ [-Wimplicit-function-declaration]
     rb_enc_register("GB2312", rb_enc_find("EUC-KR"));
                               ^~~~~~~~~~~
/tmp/ruby/v2/src/trunk-gc-asserts/enc/gb2312.c:6:31: warning: passing argument 2 of ‘rb_enc_register’ makes pointer from integer without a cast [-Wint-conversion]
<command-line>:0:19: note: expected ‘OnigEncoding {aka const struct OnigEncodingTypeST *}’ but argument is of type ‘int’
/tmp/ruby/v2/src/trunk-gc-asserts/regenc.h:231:12: note: in expansion of macro ‘ONIG_ENC_REGISTER’
 extern int ONIG_ENC_REGISTER(const char *, OnigEncoding);
            ^~~~~~~~~~~~~~~~~
```

2 files changed, 34 insertions, 0 deletions

diff --git a/enc/depend b/enc/depend
index 0efe565a44..8bb49eb696 100644
--- a/enc/depend
+++ b/enc/depend
@@ -207,13 +207,19 @@ enc/cp949.$(OBJEXT): enc/cp949.c
 enc/cp949.$(OBJEXT): missing.h
 enc/cp949.$(OBJEXT): onigmo.h
 enc/cp949.$(OBJEXT): oniguruma.h
+enc/emacs_mule.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/emacs_mule.$(OBJEXT): $(top_srcdir)/regenc.h
+enc/emacs_mule.$(OBJEXT): $(top_srcdir)/regint.h
+enc/emacs_mule.$(OBJEXT): backward.h
 enc/emacs_mule.$(OBJEXT): config.h
 enc/emacs_mule.$(OBJEXT): defines.h
 enc/emacs_mule.$(OBJEXT): enc/emacs_mule.c
+enc/emacs_mule.$(OBJEXT): intern.h
 enc/emacs_mule.$(OBJEXT): missing.h
 enc/emacs_mule.$(OBJEXT): onigmo.h
 enc/emacs_mule.$(OBJEXT): oniguruma.h
+enc/emacs_mule.$(OBJEXT): st.h
+enc/emacs_mule.$(OBJEXT): subst.h
 enc/encdb.$(OBJEXT): $(hdrdir)/ruby.h
 enc/encdb.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/encdb.$(OBJEXT): $(top_srcdir)/internal.h
@@ -231,15 +237,21 @@ enc/encdb.$(OBJEXT): onigmo.h
 enc/encdb.$(OBJEXT): oniguruma.h
 enc/encdb.$(OBJEXT): st.h
 enc/encdb.$(OBJEXT): subst.h
+enc/euc_jp.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/euc_jp.$(OBJEXT): $(top_srcdir)/regenc.h
+enc/euc_jp.$(OBJEXT): $(top_srcdir)/regint.h
+enc/euc_jp.$(OBJEXT): backward.h
 enc/euc_jp.$(OBJEXT): config.h
 enc/euc_jp.$(OBJEXT): defines.h
 enc/euc_jp.$(OBJEXT): enc/euc_jp.c
 enc/euc_jp.$(OBJEXT): enc/jis/props.h
 enc/euc_jp.$(OBJEXT): enc/jis/props.kwd
+enc/euc_jp.$(OBJEXT): intern.h
 enc/euc_jp.$(OBJEXT): missing.h
 enc/euc_jp.$(OBJEXT): onigmo.h
 enc/euc_jp.$(OBJEXT): oniguruma.h
+enc/euc_jp.$(OBJEXT): st.h
+enc/euc_jp.$(OBJEXT): subst.h
 enc/euc_kr.$(OBJEXT): $(top_srcdir)/regenc.h
 enc/euc_kr.$(OBJEXT): config.h
 enc/euc_kr.$(OBJEXT): defines.h
@@ -261,13 +273,20 @@ enc/gb18030.$(OBJEXT): enc/gb18030.c
 enc/gb18030.$(OBJEXT): missing.h
 enc/gb18030.$(OBJEXT): onigmo.h
 enc/gb18030.$(OBJEXT): oniguruma.h
+enc/gb2312.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/gb2312.$(OBJEXT): $(top_srcdir)/regenc.h
+enc/gb2312.$(OBJEXT): assert.h
+enc/gb2312.$(OBJEXT): backward.h
 enc/gb2312.$(OBJEXT): config.h
 enc/gb2312.$(OBJEXT): defines.h
 enc/gb2312.$(OBJEXT): enc/gb2312.c
+enc/gb2312.$(OBJEXT): encoding.h
+enc/gb2312.$(OBJEXT): intern.h
 enc/gb2312.$(OBJEXT): missing.h
 enc/gb2312.$(OBJEXT): onigmo.h
 enc/gb2312.$(OBJEXT): oniguruma.h
+enc/gb2312.$(OBJEXT): st.h
+enc/gb2312.$(OBJEXT): subst.h
 enc/gbk.$(OBJEXT): $(top_srcdir)/regenc.h
 enc/gbk.$(OBJEXT): config.h
 enc/gbk.$(OBJEXT): defines.h
@@ -404,16 +423,22 @@ enc/koi8_u.$(OBJEXT): enc/koi8_u.c
 enc/koi8_u.$(OBJEXT): missing.h
 enc/koi8_u.$(OBJEXT): onigmo.h
 enc/koi8_u.$(OBJEXT): oniguruma.h
+enc/shift_jis.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/shift_jis.$(OBJEXT): $(top_srcdir)/regenc.h
+enc/shift_jis.$(OBJEXT): $(top_srcdir)/regint.h
+enc/shift_jis.$(OBJEXT): backward.h
 enc/shift_jis.$(OBJEXT): config.h
 enc/shift_jis.$(OBJEXT): defines.h
 enc/shift_jis.$(OBJEXT): enc/jis/props.h
 enc/shift_jis.$(OBJEXT): enc/jis/props.kwd
 enc/shift_jis.$(OBJEXT): enc/shift_jis.c
 enc/shift_jis.$(OBJEXT): enc/shift_jis.h
+enc/shift_jis.$(OBJEXT): intern.h
 enc/shift_jis.$(OBJEXT): missing.h
 enc/shift_jis.$(OBJEXT): onigmo.h
 enc/shift_jis.$(OBJEXT): oniguruma.h
+enc/shift_jis.$(OBJEXT): st.h
+enc/shift_jis.$(OBJEXT): subst.h
 enc/trans/big5.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/trans/big5.$(OBJEXT): $(top_srcdir)/transcode_data.h
 enc/trans/big5.$(OBJEXT): assert.h
@@ -752,14 +777,21 @@ enc/windows_1257.$(OBJEXT): enc/iso_8859.h
 enc/windows_1257.$(OBJEXT): enc/windows_1257.c
 enc/windows_1257.$(OBJEXT): missing.h
 enc/windows_1257.$(OBJEXT): onigmo.h
+enc/windows_31j.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 enc/windows_31j.$(OBJEXT): $(top_srcdir)/regenc.h
+enc/windows_31j.$(OBJEXT): $(top_srcdir)/regint.h
+enc/windows_31j.$(OBJEXT): backward.h
 enc/windows_31j.$(OBJEXT): config.h
 enc/windows_31j.$(OBJEXT): defines.h
 enc/windows_31j.$(OBJEXT): enc/jis/props.h
 enc/windows_31j.$(OBJEXT): enc/jis/props.kwd
+enc/windows_31j.$(OBJEXT): enc/shift_jis.c
 enc/windows_31j.$(OBJEXT): enc/shift_jis.h
 enc/windows_31j.$(OBJEXT): enc/windows_31j.c
+enc/windows_31j.$(OBJEXT): intern.h
 enc/windows_31j.$(OBJEXT): missing.h
 enc/windows_31j.$(OBJEXT): onigmo.h
 enc/windows_31j.$(OBJEXT): oniguruma.h
+enc/windows_31j.$(OBJEXT): st.h
+enc/windows_31j.$(OBJEXT): subst.h
 # AUTOGENERATED DEPENDENCIES END
diff --git a/enc/gb2312.c b/enc/gb2312.c
index 717ad308da..6fc15735fc 100644
--- a/enc/gb2312.c
+++ b/enc/gb2312.c
@@ -1,3 +1,5 @@
+#include <ruby/ruby.h>
+#include <ruby/encoding.h>
 #include "regenc.h"
 
 void