syslogd: Block timezone updating in capability mode

Do not update local timezone data upon reload when syslogd is compiled
with libcasper support. Timezone files are pre-fetched using the
caph_cache_tzdata() function before entering capability mode. Attempts
to re-fetch these files will result in a capability violation.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41466