ruby.h: parent in rb_data_type_t

* ruby.h (rb_data_type_t): revert r48647 and revise parent member.
  [ruby-core:66969] [Bug #10621]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48962 b2dd03c8-39d4-4d8f-98ff-823fe69b080e