avcodec/h264_metadata_bsf: Improve included headers

h264_sei.h is no longer used since the SEIs were moved to sei.h;
this also avoids inclusions of avcodec.h and bytestream.h.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h264_metadata_bsf.c

@@ -26,10 +26,11 @@
 #include "cbs.h"
 #include "cbs_bsf.h"
 #include "cbs_h264.h"
+#include "cbs_sei.h"
 #include "h264.h"
 #include "h264_levels.h"
-#include "h264_sei.h"
 #include "h2645data.h"
+#include "sei.h"
 
 enum {
     FLIP_HORIZONTAL = 1,