Add hostname dep in lightdm.service

Start lightdm after systemd-hostnamed.service by adding an appropriate
line to /lib/systemd/system/lightdm.service.

Fix #291

debian/lightdm.service

@@ -2,7 +2,7 @@
 Description=Light Display Manager
 Documentation=man:lightdm(1)
 Conflicts[email protected] plymouth-quit.service
-After=systemd-user-sessions.service [email protected] plymouth-quit.service
+After=systemd-user-sessions.service [email protected] plymouth-quit.service systemd-hostnamed.service
 
 [Service]
 # temporary safety check until all DMs are converted to correct