Fix for libical >= 4

Index: res/res_calendar_icalendar.c
--- res/res_calendar_icalendar.c.orig
+++ res/res_calendar_icalendar.c
@@ -341,7 +341,7 @@ static void icalendar_add_event(icalcomponent *comp, s
 	} else { /* Offset from either dtstart or dtend */
 		/* XXX Technically you can check RELATED to see if the event fires from the END of the event
 		 * But, I'm not sure I've ever seen anyone implement it in calendaring software, so I'm ignoring for now */
-		tmp = icaltime_add(start, trigger.duration);
+		tmp = icalduration_extend(start, trigger.duration);
 		event->alarm = icaltime_as_timet_with_zone(tmp, icaltime_get_timezone(start));
 	}
 
