* gc.c (gc_mark_children): STR_ASSOC is no longer available.
  Reported by @nagachika.
  http://d.hatena.ne.jp/nagachika/20140204



git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44823 b2dd03c8-39d4-4d8f-98ff-823fe69b080e