Fix for libical >= 4

Index: res/res_calendar_caldav.c
--- res/res_calendar_caldav.c.orig
+++ res/res_calendar_caldav.c
@@ -349,7 +349,7 @@ static time_t icalfloat_to_timet(icaltimetype time)
  * span here, and instead will grab the start and end from the component, which will
  * allow us to test for floating times or dates.
  */
-static void caldav_add_event(icalcomponent *comp, struct icaltime_span *span, void *data)
+static void caldav_add_event(icalcomponent *comp, const struct icaltime_span *span, void *data)
 {
 	struct caldav_pvt *pvt = data;
 	struct ast_calendar_event *event;
@@ -464,7 +464,7 @@ static void caldav_add_event(icalcomponent *comp, stru
 	} 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));
 	}
 
