Explicitly set test_proto.proto's syntax.

dbus/test_proto.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+syntax = "proto2";
+
 option optimize_for = LITE_RUNTIME;
 
 // This is a simple dummy protocol buffer that is used for testing handling of
 // protocol buffers in MessageReader and MessageWriter.
 
 message TestProto {
     optional string text = 1;
     optional int32 number = 2;
-}
+}