OpenBSD has getentropy, but no sys/random.h

https://man.openbsd.org/getentropy

Try to fix https://rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20210118T023008Z.fail.html.gz
```
compiling random.c
random.c:53:11: fatal error: 'sys/random.h' file not found
# include <sys/random.h>
          ^~~~~~~~~~~~~~
1 error generated.
```

2 files changed, 4 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index f672f6d7f7..e4bdf9fb71 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1195,6 +1195,7 @@ AC_CHECK_HEADERS(sys/ioctl.h)
 AC_CHECK_HEADERS(sys/mkdev.h)
 AC_CHECK_HEADERS(sys/param.h)
 AC_CHECK_HEADERS(sys/prctl.h)
+AC_CHECK_HEADERS(sys/random.h)
 AC_CHECK_HEADERS(sys/resource.h)
 AC_CHECK_HEADERS(sys/select.h)
 AC_CHECK_HEADERS(sys/sendfile.h)
diff --git a/random.c b/random.c
index 7b78efe7ca..c11cd803f2 100644
--- a/random.c
+++ b/random.c
@@ -50,7 +50,9 @@
 #endif
 
 #if defined HAVE_GETRANDOM || defined HAVE_GETENTROPY
-# include <sys/random.h>
+# if defined(HAVE_SYS_RANDOM_H)
+#  include <sys/random.h>
+# endif
 #elif defined __linux__ && defined __NR_getrandom
 # include <linux/random.h>
 #endif