Suppress format warnings

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2621 f4f36d40-9719-0410-9868-dde021a23eda
Use class_is_cocoa_class_p() in osx_mf_objc_classnames()

fix missing commit change of function osx_mf_objc_classnames() at r2620.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2622 f4f36d40-9719-0410-9868-dde021a23eda