Add internal.h include for av_read_frame_flush prototype

Fixes build with clang

Originally committed as revision 22273 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/seek.c

@@ -22,6 +22,7 @@
 
 #include "seek.h"
 #include "libavutil/mem.h"
+#include "internal.h"
 
 // NOTE: implementation should be moved here in another patch, to keep patches
 // separated.