Enable the -Wundef warning for clang

webrtc/modules/audio_processing/transient/dyadic_decimator_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.
  */
 
 #include "webrtc/modules/audio_processing/transient/dyadic_decimator.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 static const size_t kEvenBufferLength = 6;
 static const size_t kOddBufferLength = 5;
 static const size_t kOutBufferLength = 3;
 
 int16_t const test_buffer_even_len[] = {0, 1, 2, 3, 4, 5};
 int16_t const test_buffer_odd_len[]  = {0, 1, 2, 3, 4};
 int16_t test_buffer_out[kOutBufferLength];
@@ skipping 93 matching lines @@
                                       test_buffer_out,
                                       kOutBufferLength);
 
   EXPECT_EQ(expected_out_samples, out_samples);
 
   EXPECT_EQ(1, test_buffer_out[0]);
   EXPECT_EQ(3, test_buffer_out[1]);
 }
 
 }  // namespace webrtc