Add missing include.

The functions "memcpy" and "memset" are defined in "string.h" which
was not included. Found this when compiling with g++ 5.4 on Ubuntu
Xenial.

Bug: None
Change-Id: Ife9a9ce2a168ecc24d983afcfc0a39784cbedf9f
Reviewed-on: https://webrtc-review.googlesource.com/54121
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22043}
diff --git a/api/audio/audio_frame.cc b/api/audio/audio_frame.cc
index 3dc510d..108a523 100644
--- a/api/audio/audio_frame.cc
+++ b/api/audio/audio_frame.cc
@@ -8,6 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <string.h>
+
 #include "api/audio/audio_frame.h"
 
 #include "rtc_base/checks.h"