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:

1 job for master in 4 minutes and 21 seconds (queued for 11 seconds)
Status Job ID Name Coverage
passed #6412571