Enable the -Wundef warning for clang

webrtc/modules/audio_coding/neteq/comfort_noise_unittest.cc

 /*
  *  Copyright (c) 2012 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 ComfortNoise class.
 
 #include "webrtc/modules/audio_coding/neteq/comfort_noise.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/neteq/mock/mock_decoder_database.h"
 #include "webrtc/modules/audio_coding/neteq/sync_buffer.h"
 
 namespace webrtc {
 
 TEST(ComfortNoise, CreateAndDestroy) {
   int fs = 8000;
   MockDecoderDatabase db;
   SyncBuffer sync_buffer(1, 1000);
   ComfortNoise cn(fs, &db, &sync_buffer);
   EXPECT_CALL(db, Die());  // Called when |db| goes out of scope.
 }
 
 // TODO(hlundin): Write more tests.
 
 }  // namespace webrtc