Enable the -Wundef warning for clang

webrtc/modules/video_coding/nack_module_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 <cstring>
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/video_coding/include/video_coding_defines.h"
 #include "webrtc/modules/video_coding/nack_module.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 class TestNackModule : public ::testing::Test,
                        public NackSender,
                        public KeyFrameRequestSender {
  protected:
   TestNackModule()
@@ skipping 283 matching lines @@
   clock_->AdvanceTimeMilliseconds(100);
   nack_module_.Process();
   packet.seqNum = 3;
   EXPECT_EQ(3, nack_module_.OnReceivedPacket(packet));
   packet.seqNum = 4;
   EXPECT_EQ(3, nack_module_.OnReceivedPacket(packet));
   EXPECT_EQ(0, nack_module_.OnReceivedPacket(packet));
 }
 
 }  // namespace webrtc