check_ntp_time.c: As part of #661, also add stratum warn/crit to usage/help text

plugins/check_ntp_time.c

@@ -722,6 +722,6 @@ void
 print_usage(void)
 {
 	printf ("%s\n", _("Usage:"));
-	printf(" %s -H <host> [-4|-6] [-w <warn>] [-c <crit>] [-v verbose] [-o <time offset>] [-d <delay>]\n", progname);
+	printf(" %s -H <host> [-4|-6] [-w <warn>] [-c <crit>] [-v verbose] [-o <time offset>] [-d <delay>] [-W <stratum warn>] [-C <stratum crit>]\n", progname);
 }