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
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42875