Set reboot notifications as urgent to avoid them timing out (Fixes: #341)

It was confirmed in https://bugzilla.gnome.org/show_bug.cgi?id=783719
that this field will influence the timeout
1 job for wip/superm1/reboot-popup-timeout in 4 minutes and 12 seconds (queued for 32 minutes and 14 seconds)
latest
Status Job ID Name Coverage
  Build
passed #103556
build-gnome-software

00:04:12