Get rid of the pre-C11 _Alignof define in zic.c

Bertrand Drouvot <bertranddrouvot.pg@gmail.com> writes:

Now that f5e0186f865c is in, I think that we can get rid of this pre-C11 define
and just rely on the C11 _Alignof definition. That's done in the attached.

I'm not in favor of introducing random discrepancies from tzdb
upstream.

(We are well overdue for re-syncing this code with recent tzdb,
but I don't think changes like this one will make that any less
painful.)

regards, tom lane