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

Hi hackers,

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.

Note that e6be84356bbb already introduced the usage of _Noreturn in it.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com