Add missing sys module when referencing stderr

`repo cherry-pick` was broken because we were referencing stderr
instead of sys.stderr.  This should fix it.

Change-Id: I67f25c3a0790d029edc65732c319df7c684546c8

subcmds/cherry_pick.py

@@ -83,8 +83,8 @@ change id will be added.
     else:
       print('NOTE: When committing (please see above) and editing the commit'
             'message, please remove the old Change-Id-line and add:')
-      print(self._GetReference(sha1), file=stderr)
-      print(file=stderr)
+      print(self._GetReference(sha1), file=sys.stderr)
+      print(file=sys.stderr)
 
   def _IsChangeId(self, line):
     return CHANGE_ID_RE.match(line)