io.c (rb_f_backquote): trade volatile for manual recycle

* io.c (rb_f_backquote): trade volatile for manual recycle
  rb_gc_force_recycle ensures object is visible until recycle

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44995 b2dd03c8-39d4-4d8f-98ff-823fe69b080e