res_pjsip_outbound_publishing: After unloading the library won't load again

The same thing was happening in res_pjsip_publish_asterisk. When the library
was unloaded it did not unregister the object type from sorcery. Subsequent
loads resulted in a failed load due to the sorcery type already existing.

Change-Id: Ifdc25e94e4cd40bc5a19eb4d0a00b86c2e9fedc9