Enable the -Wundef warning for clang

webrtc/modules/audio_coding/acm2/call_statistics_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/acm2/call_statistics.h"
 
 namespace webrtc {
 
 namespace acm2 {
 
 TEST(CallStatisticsTest, InitializedZero) {
   CallStatistics call_stats;
   AudioDecodingCallStats stats;
 
@@ skipping 26 matching lines @@
   EXPECT_EQ(1, stats.decoded_plc_cng);
   EXPECT_EQ(1, stats.decoded_muted_output);
 }
 
 }  // namespace acm2
 
 }  // namespace webrtc