Enable the -Wundef warning for clang

webrtc/modules/audio_coding/neteq/dsp_helper_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.
  */
 
 #include "webrtc/modules/audio_coding/neteq/dsp_helper.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/neteq/audio_multi_vector.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 TEST(DspHelper, RampSignalArray) {
   static const int kLen = 100;
   int16_t input[kLen];
   int16_t output[kLen];
   // Fill input with 1000.
@@ skipping 56 matching lines @@
     }
   }
   // Verify the last |kLen| samples are left untouched.
   for (; i < 3 * kLen; ++i) {
     for (int channel = 0; channel < kChannels; ++channel) {
       EXPECT_EQ(1000, input[channel][i]);
     }
   }
 }
 }  // namespace webrtc