Added Xscreensaver to XFCE list so users can lock the screen

Everytime I installed Antergos I was not able to lock the screen using XFCE.

The cause seems to be that there is one package missing as shown on this [link](https://wiki.archlinux.org/index.php/xfce#Lock_the_screen). I've Chosen Xscreensaver because it is the most simple and usually used on this DE.

Thanks!