glib: Fix cast from 'GTime *' (aka 'int *') to 'time_t *' (aka 'long *')
Given by clang on my system.
Sounds rather scary since then we're storing in poppler_date_parse a bigger value than what really fits.
Fixed by the suggestion of https://developer.gnome.org/glib/stable/glib-Date-and-Time-Functions.html#GTime
Changing the type of _PopplerAttachment ctime/mtime would change the structure size, thus break the BC, so leaving that for the future for now