Add missing #include "libavutil/avstring.h", fixes

libavcodec/utils.c:1139: warning: implicit declaration of function 'av_strlcat'

Originally committed as revision 15777 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -28,6 +28,7 @@
 /* needed for mkstemp() */
 #define _XOPEN_SOURCE 600
 
+#include "libavutil/avstring.h"
 #include "libavutil/integer.h"
 #include "libavutil/crc.h"
 #include "avcodec.h"