matroskamux: Include end padding in the block duration for Opus streams
It has to be included in the block duration but in GStreamer we're not including it in the buffer duration, so it has to be added again here.
Not including it in the block duration can lead to fatal errors when playing back with Firefox if there are more padding samples than actual samples, e.g.
D/MediaDemuxer WebMDemuxer[7f6a0808b900] ::GetNextPacket: Padding frames larger than packet size, flagging the packet for error (padding: {13500000,1000000000}, duration: {6000,1000000}, already processed: false)