util-lib: add (void) cast to indicate that we don't care about the normalization success

After all, we verify that every calendar part is not out of bounds later on,
and it's fully OK if the normalization has no effect.

src/basic/calendarspec.c

@@ -978,7 +978,7 @@ static int find_next(const CalendarSpec *spec, struct tm *tm, usec_t *usec) {
 
         for (;;) {
                 /* Normalize the current date */
-                mktime_or_timegm(&c, spec->utc);
+                (void) mktime_or_timegm(&c, spec->utc);
                 c.tm_isdst = -1;
 
                 c.tm_year += 1900;