Enable the -Wundef warning for clang

webrtc/modules/audio_coding/neteq/post_decode_vad_unittest.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // Unit tests for PostDecodeVad class.
 
 #include "webrtc/modules/audio_coding/neteq/post_decode_vad.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 TEST(PostDecodeVad, CreateAndDestroy) {
   PostDecodeVad vad;
 }
 
 // TODO(hlundin): Write more tests.
 
 }  // namespace webrtc