Ignore prepended modules when looking for already defined methods on a
class to set the visibility on.
[Fix GH-1834]

From: Dylan Thacker-Smith <Dylan.Smith@shopify.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e