xwayland: Use libxcvt

Xwayland is using a copy of the CVT generator found in Xorg.

Rather than duplicating the code within the xserver tree, use the
libxcvt implementation instead.

Closes: xorg/xserver#1142


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 6ea2c001)