avcodec/audiotoolboxnec: set set keyframe flag in output packets

Don't depend on the generic code setting this.
This is in preparation for a following change.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/audiotoolboxenc.c

@@ -576,6 +576,7 @@ static int ffat_encode(AVCodecContext *avctx, AVPacket *avpkt,
                                      avctx->frame_size,
                            &avpkt->pts,
                            &avpkt->duration);
+        avpkt->flags |= AV_PKT_FLAG_KEY;
     } else if (ret && ret != 1) {
         av_log(avctx, AV_LOG_ERROR, "Encode error: %i\n", ret);
         return AVERROR_EXTERNAL;