Fix memory leak for iclass

[Bug #19550]

If !RCLASS_EXT_EMBEDDED (e.g. 32 bit systems) then the rb_classext_t is
allocated throug malloc so it must be freed.

The issue can be seen in the following script:

```
20.times do
  100_000.times do
    mod = Module.new
    Class.new do
      include mod
    end
  end

  # Output the Resident Set Size (memory usage, in KB) of the current Ruby process
  puts `ps -o rss= -p #{$$}`
end
```

Before this fix, the max RSS is 280MB, while after this change, it's
30MB.

1 files changed, 1 insertions, 1 deletions

diff --git a/gc.c b/gc.c
index af789bc9ee..c03738986c 100644
--- a/gc.c
+++ b/gc.c
@@ -3677,7 +3677,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
         cc_table_free(objspace, obj, FALSE);
         rb_class_remove_from_module_subclasses(obj);
         rb_class_remove_from_super_subclasses(obj);
-#if !USE_RVARGC
+#if !RCLASS_EXT_EMBEDDED
         xfree(RCLASS_EXT(obj));
 #endif