timezone: Load GisLocationEntry type before page template

Without this, constructing the timezone page fails and everything goes
south.

> (gnome-initial-setup:2071): Gtk-CRITICAL **: 18:06:35.097: Error
> building template class 'GisTimezonePage' for an instance of type
> 'GisTimezonePage': .:3:1233 Invalid object type 'GisLocationEntry'

Fixes: a7dfea20 "timezone: Reintroduce GisLocationEntry"
Part-of: <https://gitlab.gnome.org/GNOME/gnome-initial-setup/-/merge_requests/138>