lightdm: Order service correctly after startup-tags.service

Without specifying the ordering in regards to `startup-tags.service` it
is not guaranteed, that `/node/tags/` has been populated yet, and we
have observed a non-starting LightDM on desktop systems in practice. So,
order the service unit correctly.

lightdm.be0

@@ -81,8 +81,8 @@ EOF
   cat <<-UNIT > ${D}/etc/systemd/system/lightdm.service
 	[Unit]
 	Description=LightDM
-	Wants=nvidia.service
-	After=nvidia.service
+	Wants=nvidia.service startup-tags.service
+	After=nvidia.service startup-tags.service
 	ConditionPathExists=/node/tags/lightdm
 
 	[Service]