Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_packet/compound_packet_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 "webrtc/modules/rtp_rtcp/source/rtcp_packet/compound_packet.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet/bye.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet/fir.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet/receiver_report.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet/sender_report.h"
 #include "webrtc/test/rtcp_packet_parser.h"
 
 using webrtc::rtcp::Bye;
 using webrtc::rtcp::CompoundPacket;
 using webrtc::rtcp::Fir;
@@ skipping 128 matching lines @@
 
     int packets_created_ = 0;
   } verifier;
   const size_t kBufferSize = kRrLength + kReportBlockLength;
   uint8_t buffer[kBufferSize];
   EXPECT_TRUE(compound.BuildExternalBuffer(buffer, kBufferSize, &verifier));
   EXPECT_EQ(2, verifier.packets_created_);
 }
 
 }  // namespace webrtc