avcodec/videotoolbox: add internal.h for header depenedency

Fixes build failures for videotoolbox

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/videotoolbox.c

@@ -29,6 +29,7 @@
 #include "libavutil/pixdesc.h"
 #include "bytestream.h"
 #include "decode.h"
+#include "internal.h"
 #include "h264dec.h"
 #include "hevcdec.h"
 #include "mpegvideo.h"