lavf/avformat.h: drop the avcodec.h include

Since AVStream.codec is gone, avformat.h does not require anything from
avcodec.h.

libavformat/avformat.h

@@ -309,7 +309,12 @@
 
 #include <time.h>
 #include <stdio.h>  /* FILE */
-#include "libavcodec/avcodec.h"
+
+#include "libavcodec/codec.h"
+#include "libavcodec/codec_par.h"
+#include "libavcodec/defs.h"
+#include "libavcodec/packet.h"
+
 #include "libavutil/dict.h"
 #include "libavutil/log.h"