avcodec/mathops: add missing header for ff_reverse

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/mathops.h

@@ -25,6 +25,7 @@
 #include <stdint.h>
 
 #include "libavutil/common.h"
+#include "libavutil/internal.h"
 #include "config.h"
 
 #define MAX_NEG_CROP 1024