audio/opus: update to 1.4

This new release brings the following improvements:
- Improved tuning of the Opus in-band FEC (LBRR)
- Added an OPUS_SET_INBAND_FEC(2) option that turns on FEC, but does not
  force SILK mode (FEC will be disabled in CELT mode)
- Improved tuning and various fixes to DTX

PR:		270996