Fix for libical >= 4

Index: src/calendar_ical.c
--- src/calendar_ical.c.orig
+++ src/calendar_ical.c
@@ -56,7 +56,7 @@ gint i;
 
     for (i = 0; i < N_SEARCH_PATHS; i++) {
         if (!g_access (tz_search_paths[i], F_OK | R_OK)) {
-            set_zone_directory (tz_search_paths[i]);
+            icaltimezone_set_zone_directory (tz_search_paths[i]);
             break;
         }
     }
