parse.y: use rb_fstring_new

* parse.y (rb_id2str): use rb_fstring_new() instead of rb_fstring()
  with a new string instance.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e