Building poppler creates useless change in pdfsig.pot (updates POT-Creation-Date)

I don't know if it's happening to other people (haven't heard anyone complain), but everytime I build Poppler a useless change in pdfsig.pot appears which is tracked by Git, which means I need to manually discard it before pushing a MR or before changing to another branch locally, which happens more often.

No translations have been modified or added, so this is unsolicited git work. The main hurdle is the POT-Creation-Date field which xgettext likes to update with every run.

I'm sending a MR with a fix.