GUI: Make /xban_gui only usable by players with ban priv.

gui.lua

@@ -128,6 +128,7 @@ end)
 minetest.register_chatcommand("xban_gui", {
 	description = "Show XBan GUI",
 	params = "",
+	privs = { ban=true, },
 	func = function(name, params)
 		minetest.show_formspec(name, FORMNAME, make_fs(name))
 	end,