avformat/matroskaenc: print a warning when the relative timestamp wouldnt fit in 16bit

This is somewhat unusual so its better to use warning level than debug
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskaenc.c

@@ -1712,7 +1712,7 @@ static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_
     if (mkv->cluster_pos != -1) {
         int64_t cluster_time = ts - mkv->cluster_pts + mkv->tracks[pkt->stream_index].ts_offset;
         if ((int16_t)cluster_time != cluster_time) {
-            av_log(s, AV_LOG_DEBUG, "Starting new cluster due to timestamp\n");
+            av_log(s, AV_LOG_WARNING, "Starting new cluster due to timestamp\n");
             mkv_start_new_cluster(s, pkt);
         }
     }