Add const qualifier to tzn returned by timestamp2tm()

Peter Eisentraut <peter_e@gmx.net>

Commit: eb990a2b9efff853d0c68a9cf902b1df23498540
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2012-03-15T19:17:19Z
Releases: 9.2.0
Add const qualifier to tzn returned by timestamp2tm()

The tzn value might come from tm->tm_zone, which libc declares as
const, so it's prudent that the upper layers know about this as well.

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