xserver_xorg-server: fix fontrootdir configuration

The --with-fontdir switch has been substituted by the --with-fontrootdir
switch a few versions ago. The use of the old configuration switch was
causing error messages like the following:

_FontTransOpen: Unable to Parse address ${prefix}/share/fonts/X11/*/
[dix] Could not init font path element ${prefix}/share/fonts/X11/*/, removing from list!

Signed-off-by: Filippo Sironi <filippo.sironi@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>