res_pjsip_geolocation: Add support for Geolocation loc-src parameter

res_pjsip_geolocation: Add support for Geolocation loc-src parameter

This adds support for the Geolocation 'loc-src' parameter to res_pjsip_geolocation.
The already existing config option 'location_source` in res_geolocation is documented to add a 'loc-src' parameter containing a user-defined FQDN to the 'Geolocation:' header,
but that option had no effect as it was not implemented by res_pjsip_geolocation.

If the `location_source` configuration option is not set or invalid, that parameter will not be added (this is already checked by res_geolocation).

This commits adds already documented functionality.