IWYU: Include <string.h> for memcpy(3) after bbf21a3fd.

Commit bbf21a3fd617abb37567a86e65f8ba18b8d64eb2 ("Remove dependencies on
modules:module_api from AudioProcessing") causes the build to fail with
libstdc++ due to several files using memcpy(3) or memset(3) while relying on
string.h being included implicitly by other headers.

Bug: webrtc:9139
Change-Id: Ib73284962f8694d8bed0551968265bfd13cab967
Reviewed-on: https://webrtc-review.googlesource.com/70180
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#22895}

api/audio/audio_frame.cc

@@ -10,6 +10,8 @@
 
 #include "api/audio/audio_frame.h"
 
+#include <string.h>
+
 #include "rtc_base/checks.h"
 #include "rtc_base/timeutils.h"
 

audio/utility/audio_frame_operations.cc

@@ -10,6 +10,7 @@
 
 #include "audio/utility/audio_frame_operations.h"
 
+#include <string.h>
 #include <algorithm>
 
 #include "rtc_base/checks.h"

common_audio/wav_header_unittest.cc

@@ -8,6 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <string.h>
 #include <limits>
 
 #include "common_audio/wav_header.h"

modules/audio_processing/vad/standalone_vad.cc

@@ -10,6 +10,8 @@
 
 #include "modules/audio_processing/vad/standalone_vad.h"
 
+#include <string.h>
+
 #include "audio/utility/audio_frame_operations.h"
 #include "rtc_base/checks.h"
 #include "typedefs.h"  // NOLINT(build/include)

modules/audio_processing/vad/vad_audio_proc.cc

@@ -12,6 +12,7 @@
 
 #include <math.h>
 #include <stdio.h>
+#include <string.h>
 
 #include "common_audio/fft4g.h"
 #include "modules/audio_processing/vad/pitch_internal.h"