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