debian: put mod_siren in non-free

Despite the relatively-generous patent grant from Polycom, the
requirement to execute further agreements to use the code and patents
means this module fails the DFSG.

We'll need to exclude this code from the source package as well.

debian/control-modules

@@ -310,6 +310,7 @@ Description: mod_silk
 Module: codecs/mod_siren
 Description: mod_siren
  Adds mod_siren.
+Section: non-free/comm
 
 Module: codecs/mod_skel_codec
 Description: mod_skel_codec