Explicitly set test_proto.proto's syntax.

dbus/test_proto.proto

Index: dbus/test_proto.proto
diff --git a/dbus/test_proto.proto b/dbus/test_proto.proto
index 0a1d06f7ff51c46b7342d8b04a6ce4d2cf251bfb..1ec128bf609a78a8d56f66318dcb650d56dd8773 100644
--- a/dbus/test_proto.proto
+++ b/dbus/test_proto.proto
@@ -2,6 +2,8 @@
 // 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
@@ -10,4 +12,4 @@ option optimize_for = LITE_RUNTIME;
 message TestProto {
     optional string text = 1;
     optional int32 number = 2;
-}
+}