Find wayland.xml from wayland-scanner.pc

In wayland, it's the "scanner" meson option that controls whether wayland.xml is installed, not the "libraries" option (which enables wayland-client and wayland-server). This means that, if wayland-scanner is installed into one path, and wayland-client and wayland-server are installed into another (as we'd like to do in Nixpkgs), it's wayland-scanner's pkgdatadir that will contain wayland.xml.