gc.c: suppress warning

* gc.c (obj_free): suppress a false shorten-64-to-32 warning,
  RUBY_TYPED_FREE_IMMEDIATELY never exceed the limit of int.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43516 b2dd03c8-39d4-4d8f-98ff-823fe69b080e