gdm: replace 'WantedBy=graphical.target' by 'Alias=display-manager.service'

Found at lxdm. Error message was there
| systemd[1]: Cannot add dependency job for unit display-manager.service ...

Inspired by [1]

[1] https://bbs.archlinux.org/viewtopic.php?pid=1131267



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>