avcodec/libdavs2: unbreak compilation failure

FF_CODEC_CAP_AUTO_THREADS added in 8a129077cc requires internal.h

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

libavcodec/libdavs2.c

@@ -23,6 +23,7 @@
  */
 
 #include "avcodec.h"
+#include "internal.h"
 #include "davs2.h"
 
 typedef struct DAVS2Context {