Fix compilation with --enable-libxvid.

Originally committed as revision 16655 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/libxvidff.c

@@ -28,6 +28,7 @@
 #include <xvid.h>
 #include <unistd.h>
 #include "avcodec.h"
+#include "libavutil/intreadwrite.h"
 #include "libxvid_internal.h"
 
 /**