Enable the -Wundef warning for clang

webrtc/common_audio/sparse_fir_filter_unittest.cc

 /*
  *  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 <memory>
 
 #include "webrtc/common_audio/sparse_fir_filter.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/arraysize.h"
 #include "webrtc/common_audio/fir_filter.h"
 
 namespace webrtc {
 namespace {
 
 static const float kCoeffs[] = {0.2f, 0.3f, 0.5f, 0.7f, 0.11f};
 static const float kInput[] =
     {1.f, 2.f, 3.f, 4.f, 5.f, 6.f, 7.f, 8.f, 9.f, 10.f};
 
@@ skipping 196 matching lines @@
                                 kSparsity,
                                 kOffset);
   filter->Filter(kInput, arraysize(kInput), output);
   sparse_filter.Filter(kInput, arraysize(kInput), sparse_output);
   for (size_t i = 0; i < arraysize(kInput); ++i) {
     EXPECT_FLOAT_EQ(output[i], sparse_output[i]);
   }
 }
 
 }  // namespace webrtc