meson: Set XCONFIGFILE to 'xorg.conf' instead of '/etc/xorg.conf'

The autoconf build hard-codes XCONFIGFILE to just 'xorg.conf':

 XF86CONFIGFILE="xorg.conf"
 AC_DEFINE_DIR(XCONFIGFILE, XF86CONFIGFILE, [Name of configuration file])

Later, the X server passes that into DoSubstitution() which expands the path:

 DoSubstitution(template="/etc/X11/%X", ..., XConfigFile="xorg.conf")

This returns "/etc/X11/xorg.conf".

The Meson build, on the other hand, sets XCONFIGFILE to
join_paths(get_option('sysconfdir'), 'xorg.conf'). If sysconfdir is /etc, this
results in '/etc/xorg.conf', resulting in DoSubstitution returning
'/etc/X11/etc/xorg.conf'.

Fix this by just hard-coding XCONFIGFILE to 'xorg.conf'.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>