xorg-server: add needed dependencies

Hi xtraemo,
We talked about that on IRC.
Without <code>xauth</code> we can't start X (unable to create .Xauthority). <code>xinit</code> provides us a way to start X, using <code>xtartx</code> (assuming we have a .xinitrc).
And if <code>xf86-input-evdev</code> isn't installed, when X is started, we can't use the keyboard.
Now, it makes the pkg really easy to install and it works almost "out-of-the-box".
Cheers :beers: