xwayland: Add new pkg-config variable for listenfd

Since commit b3f3d65e, xwayland now supports the command line option
"-listenfd" for passing file descriptors and marked "-listen" as
deprecated for this specific purpose.

Add a new pkg-config variable "have_listenfd" to the xwayland.pc so that
compositors can know this is available and use listenfd in place of the
deprecated option.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>