Enable the -Wundef warning for clang

webrtc/modules/audio_processing/level_controller/level_controller_complexity_unittest.cc

 /*
  *  Copyright (c) 2016 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 <numeric>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/array_view.h"
 #include "webrtc/base/random.h"
 #include "webrtc/modules/audio_processing/audio_buffer.h"
 #include "webrtc/modules/audio_processing/include/audio_processing.h"
 #include "webrtc/modules/audio_processing/level_controller/level_controller.h"
 #include "webrtc/modules/audio_processing/test/audio_buffer_tools.h"
 #include "webrtc/modules/audio_processing/test/bitexactness_tools.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/test/testsupport/perf_test.h"
 
@@ skipping 321 matching lines @@
         RunTogetherWithApm("LevelControlAndDefaultMobileApm", 48000, 48000,
                            capture_input_sample_rate_hz,
                            capture_output_sample_rate_hz, num_channels, true,
                            true);
       }
     }
   }
 }
 
 }  // namespace webrtc