class.c: cyclic prepend

* class.c (include_modules_at): detect cyclic prepend with original
  method table.  [ruby-core:52205] [Bug #7841]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39236 b2dd03c8-39d4-4d8f-98ff-823fe69b080e