FreeSWITCH-FS-6836

Merge pull request #347 in FS/freeswitch from ~DRAGOS_OANCEA/freeswitch-dragos:opus-dtx to master

* commit 'dc6d7c32dd433735dcc1f20a50fdebffe71817fa':
  FS-7830: mod_opus: added use-dtx param in config file (enables DTX on the encoder, announces in fmtp)