object.c: fix prepend cmp
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53380 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
object.c: fix prepend cmp
ancestor to prepended module from prepending class itself.
[ruby-core:72493] [Bug #11878]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53380 b2dd03c8-39d4-4d8f-98ff-823fe69b080e