avcodec/mlpdec: Set AV_FRAME_FLAG_KEY explicitly

It is currently always set for all audio frames, but this is
wrong (namely for MLP/TrueHD) and will be changed.

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

libavcodec/mlpdec.c

@@ -1212,6 +1212,7 @@ static int read_access_unit(AVCodecContext *avctx, AVFrame *frame,
             goto error;
         m->is_major_sync_unit = 1;
         header_size += m->major_sync_header_size;
+        frame->flags |= AV_FRAME_FLAG_KEY;
     }
 
     if (!m->params_valid) {