Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc

 /*
  *  Copyright (c) 2012 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 <list>
 #include <memory>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/modules/rtp_rtcp/source/fec_test_helper.h"
 #include "webrtc/modules/rtp_rtcp/source/forward_error_correction.h"
 #include "webrtc/modules/rtp_rtcp/source/producer_fec.h"
 
 namespace webrtc {
 
 namespace {
 constexpr uint8_t kFecPayloadType = 96;
 constexpr uint8_t kRedPayloadType = 97;
@@ skipping 173 matching lines @@
       ProducerFec::BuildRedPacket(packet->data, packet->length - kRtpHeaderSize,
                                   kRtpHeaderSize, kRedPayloadType);
   EXPECT_EQ(packet->length + kRedForFecHeaderLength, red_packet->length());
   VerifyHeader(packet->header.header.sequenceNumber,
                packet->header.header.timestamp, kRedPayloadType,
                packet->header.header.payloadType, red_packet.get(),
                true);  // Marker bit set.
 }
 
 }  // namespace webrtc