mod_sofia: Use switch_add_event_header_string() for string values.

Errors fixed:
	sofia.c: In function 'sofia_handle_sip_i_refer':
	sofia.c:7323:5: error: format not a string literal and no format arguments
	sofia.c:7324:5: error: format not a string literal and no format arguments
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

src/mod/endpoints/mod_sofia/sofia.c

@@ -7320,9 +7320,8 @@ void sofia_handle_sip_i_refer(nua_t *nua, sofia_profile_t *profile, nua_handle_t
 
 			if (switch_event_create_subclass(&event, SWITCH_EVENT_CUSTOM, MY_EVENT_ERROR)) {
 				switch_event_add_header(event, SWITCH_STACK_BOTTOM, "Error-Type", "attended_transfer");
-				switch_event_add_header(event, SWITCH_STACK_BOTTOM, "Transfer-Exten", exten);
-				switch_event_add_header(event, SWITCH_STACK_BOTTOM, "Full-Refer-To", full_ref_to);
-				
+				switch_event_add_header_string(event, SWITCH_STACK_BOTTOM, "Transfer-Exten", exten);
+				switch_event_add_header_string(event, SWITCH_STACK_BOTTOM, "Full-Refer-To", full_ref_to);
 				switch_channel_event_set_data(channel, event);
 			}