avcodec/mlpenc: Set AV_PKT_FLAG_KEY manually

TrueHD/MLP is one of the audio formats with keyframes. Currently,
the generic encoding code just sets the keyframe flag for all
returned packets, yet this is wrong for these encoders and will
be changed in a future commit. So set the flag here for those
packets that ought to have it.

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

libavcodec/mlpenc.c

@@ -2114,6 +2114,7 @@ static int mlp_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     restart_frame = !ctx->frame_index;
 
     if (restart_frame) {
+        avpkt->flags |= AV_PKT_FLAG_KEY;
         set_major_params(ctx);
         if (ctx->min_restart_interval != ctx->max_restart_interval)
             process_major_frame(ctx);