avcodec/internal: Move ff_get_buffer() to decode.h

Only used by decoders (encoders have ff_encode_alloc_frame()).

Also clean up the other headers a bit while removing now redundant
internal.h inclusions.

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

libavcodec/avrndec.c

@@ -21,7 +21,7 @@
 
 #include "avcodec.h"
 #include "codec_internal.h"
-#include "internal.h"
+#include "decode.h"
 #include "libavutil/imgutils.h"
 
 typedef struct {