display-manager: Require systemd-udev-settle.service

It was already ordered after systemd-udev-settle.service, but that
doesn't do anything if no other units require
systemd-udev-settle.service. This was causing random failures during X
server startup, e.g.

machine# [   12.691372] display-manager[607]: (EE) open /dev/dri/card0: No such file or directory

http://hydra.nixos.org/build/41062823
(cherry picked from commit e6bcff4)

nixos/modules/services/x11/xserver.nix

@@ -504,6 +504,7 @@ in
       { description = "X11 Server";
 
         after = [ "systemd-udev-settle.service" "local-fs.target" "acpid.service" "systemd-logind.service" ];
+        wants = [ "systemd-udev-settle.service" ];
 
         restartIfChanged = false;