Enable the -Wundef warning for clang

webrtc/voice_engine/voice_engine_fixture.h

 /*
  *  Copyright (c) 2015 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.
  */
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_device/include/fake_audio_device.h"
 #include "webrtc/test/mock_transport.h"
 #include "webrtc/voice_engine/include/voe_base.h"
 #include "webrtc/voice_engine/include/voe_network.h"
 #include "webrtc/voice_engine/mock/mock_voe_observer.h"
 
 namespace webrtc {
 
 class VoiceEngineFixture : public ::testing::Test {
  protected:
   VoiceEngineFixture();
   ~VoiceEngineFixture();
 
   VoiceEngine* voe_;
   VoEBase* base_;
   VoENetwork* network_;
   MockVoEObserver observer_;
   FakeAudioDeviceModule adm_;
   MockTransport transport_;
 };
 
 }  // namespace webrtc