lxdoom: make package depend on X11 server

lxdoom depends on a X11 server to be built, this patch will only show lxdoom if
a X11 server is selected and add xserver_xorg-server to the dependency list.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>

package/games/lxdoom/Config.in

@@ -1,4 +1,5 @@
 config BR2_PACKAGE_LXDOOM
 	bool "lxdoom"
+	depends on !BR2_PACKAGE_XSERVER_none
 	help
 	  This is the linux version of the popular doom game.