periodic/daily/480.leapfile-ntpd: only attempt to refresh leap-seconds.list

periodic/daily/480.leapfile-ntpd: only attempt to refresh leap-seconds.list
when ntpd is enabled.

The leap-seconds.list is used exclusively by ntpd, therefore, do not bother
to perform the fetch when ntpd is not enabled.

PR:		conf/275419
Reviewed by:	cy, michaelo, imp
Differential Revision: https://reviews.freebsd.org/D42875

(cherry picked from commit 3b3195f6767b39eb33b3523134ef988931c9c86d)
(cherry picked from commit 3ef596c6e80562710da09c16558d7351749ea143)

Security:       FreeBSD-EN-24:01.tzdata
Approved by:    so (gordon)