Move #include where it belongs.

This as a sideeffect fixes the linking failure of ffplay.

Originally committed as revision 20832 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/avstring.c

@@ -24,6 +24,7 @@
 #include <string.h>
 #include <ctype.h>
 #include "avstring.h"
+#include "mem.h"
 
 int av_strstart(const char *str, const char *pfx, const char **ptr)
 {

libavutil/avstring.h

@@ -22,7 +22,6 @@
 #define AVUTIL_AVSTRING_H
 
 #include <stddef.h>
-#include "mem.h"
 
 /**
  * Return non-zero if pfx is a prefix of str. If it is, *ptr is set to