meta: fix compiler warning in date_type_parse()

meta: fix compiler warning in date_type_parse()

After commit 0210097879 ("meta: time: use uint64_t instead of time_t")
there is a compiler warning due to comparison of the return value from
parse_iso_date with -1, which is now implicitly cast to uint64_t.

Fix this by making parse_iso_date take a pointer to the tstamp and
return bool instead.

Fixes: 0210097879 ("meta: time: use uint64_t instead of time_t")
Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>