constify RHash::ifnone.

RHash::ifnone should be protected by write-barriers so this field
should be const. However, to introduce GC.compact, the const was
removed. This commit revert this removing `const` and modify
gc.c `TYPED_UPDATE_IF_MOVED` to remove `const` forcely by a type cast.

1 files changed, 1 insertions, 1 deletions

diff --git a/internal.h b/internal.h
index b931c115df..f8baba48c1 100644
--- a/internal.h
+++ b/internal.h
@@ -873,7 +873,7 @@ struct RHash {
         struct ar_table_struct *ar; /* possibly 0 */
     } as;
     int iter_lev;
-    VALUE ifnone;
+    const VALUE ifnone;
 };
 
 #ifdef RHASH_ITER_LEV