parse.y: bare kwrest_mark

* parse.y (f_kwrest): allow bare kwrest_mark as valid syntax.  its
  semantics is still undefined.  [Bug #7662] [ruby-core:51269]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e