parse.y: disable past scope warnings

* parse.y (gettable_gen): disable warnings of possible reference
  to a local variable defined in a past scope.
  [ruby-core:67162] [Bug #10661]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49082 b2dd03c8-39d4-4d8f-98ff-823fe69b080e