tzcode: fix tz change detection logic

clock_gettime() returns 0 if it succeeds, so don't capture that into the
fail logic.  With this, WITH_DETECT_TZ_CHANGES successfully detects a
change after 61 seconds.

Reviewed by:	imp, trasz
Differential Revision:	https://reviews.freebsd.org/D33494

contrib/tzcode/stdtime/localtime.c

@@ -1278,7 +1278,7 @@ recheck_tzdata()
 	 * We want to recheck the timezone file every 61 sec.
 	 */
 	error = clock_gettime(CLOCK_MONOTONIC, &now);
-	if (error <= 0) {
+	if (error < 0) {
 		/* XXX: Can we somehow report this? */
 		return 0;
 	}