Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_utility_unittest.cc

 /*
  *  Copyright (c) 2014 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/base/checks.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/modules/rtp_rtcp/source/rtcp_utility.h"
 
 namespace webrtc {
 
 using RTCPUtility::RtcpCommonHeader;
 
 namespace rtcp {
@@ skipping 130 matching lines @@
 
   EXPECT_TRUE(RtcpParseCommonHeader(buffer, RtcpCommonHeader::kHeaderSizeBytes,
                                     &header));
   EXPECT_EQ(kCountOrFormat, header.count_or_format);
   EXPECT_EQ(kPacketType, header.packet_type);
 }
 
 }  // namespace rtcp
 }  // namespace webrtc