aft.c: Prefer localtime() over localtime_r().

While localtime(3) is required to behave as though tzset(3) was
called, localtime_r(3) does not have this requirement, and portable
code should thus call tzset(3) before localtime_r(3). This is not
the case for get_local_time() of aft.c.

As we are single-threaded here, we don't need the thread-safe version,
so it is easiest to switch to localtime(3).