Enable the -Wundef warning for clang

webrtc/modules/audio_coding/neteq/mock/mock_external_decoder_pcm16b.h

 /*
  *  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.
  */
 
 #ifndef WEBRTC_MODULES_AUDIO_CODING_NETEQ_MOCK_MOCK_EXTERNAL_DECODER_PCM16B_H_
 #define WEBRTC_MODULES_AUDIO_CODING_NETEQ_MOCK_MOCK_EXTERNAL_DECODER_PCM16B_H_
 
 #include "webrtc/modules/audio_coding/codecs/audio_decoder.h"
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/audio_coding/codecs/pcm16b/pcm16b.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 using ::testing::_;
 using ::testing::Invoke;
 
 // Implement an external version of the PCM16b decoder.
@@ skipping 63 matching lines @@
 
   int SampleRateHz() const /* override */ { return real_.SampleRateHz(); }
   size_t Channels() const /* override */ { return real_.Channels(); }
 
  private:
   ExternalPcm16B real_;
 };
 
 }  // namespace webrtc
 #endif  // WEBRTC_MODULES_AUDIO_CODING_NETEQ_MOCK_MOCK_EXTERNAL_DECODER_PCM16B_H_