Add internal.h include for av_read_frame_flush prototype

Fixes build with clang

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22273 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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.