parse.y: warn assigned but unused in toplevel

* parse.y (local_push_gen): warn assigned but unused variables also in
  toplevel, except for -e option.  [Feature #7730] [ruby-core:51580]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38982 b2dd03c8-39d4-4d8f-98ff-823fe69b080e