freetdm: Adding condition to parse sip headers only for non-freetdm channels.

         This fixes an issue where ss7 native bridge was accidentally enabled
         any time two freetdm channels were bridged regardless of the freetdm_native_sigbridge
         variable value.

libs/freetdm/mod_freetdm/mod_freetdm.c

@@ -1349,7 +1349,7 @@ static switch_call_cause_t channel_outgoing_channel(switch_core_session_t *sessi
 		chan_id = 0;
 	}
 
-	if (session && globals.sip_headers) {
+	if (session && globals.sip_headers && !switch_core_session_check_interface (session,freetdm_endpoint_interface) ) {
 		switch_channel_t *channel = switch_core_session_get_channel(session);
 		const char *sipvar;