Don't start sessions until there are users

Delay startup of VNC sessions until user databases are up and running.
This is the same ordering that GDM imposes to make sure users can
actually log in.

unix/vncserver/vncserver@.service.in

@@ -31,7 +31,7 @@
 
 [Unit]
 Description=Remote desktop service (VNC)
-After=syslog.target network.target
+After=syslog.target network.target systemd-user-sessions.service
 
 [Service]
 Type=forking