Fix volatility marking of timestamptz_trunc_zone.

Fix volatility marking of timestamptz_trunc_zone.

It's safe to mark this as immutable, because it does not depend
on the timezone GUC setting.  Oversight in commit 600b04d6b.

(There's an argument that timezone definitions do change from
time to time, but we have not worried about that in marking
other timestamp-related functions; for example AT TIME ZONE
has always been considered immutable.  The situation is no
worse than our problems with time-varying locales, surely.)

Przemysław Sztoch

Discussion: https://postgr.es/m/eaa3fabe-50fc-bbe8-b096-ce62ddadab85@sztoch.pl