manager: Do not use unnecessary volatile qualifier on GQuark

As per new GLib in CI image fprintd doesn't build anymore, since
g_once_init_enter now warns about using a volatile value, as this has
never been supposed to be the case, despite its signature [1].

Related to: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

[1] https://gitlab.gnome.org/GNOME/glib/-/issues/600
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