nscd: Use time_t for return type of addgetnetgrentX

Using int may give false results for future dates (timeouts after the
year 2028).

Fixes commit 04a21e050d64a1193a6daab872bca2528bda44b ("CVE-2024-33601,
CVE-2024-33602: nscd: netgroup: Use two buffers in addgetnetgrentX
(bug 31680)").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>