gtk-layer-shell: depend on wayland-scanner

Otherwise, wayland-scanner would be picked up from the wayland in
buildInputs, which isn't cross-friendly and will stop working when we
split wayland-scanner into a separate package.

pkgs/development/libraries/gtk-layer-shell/default.nix

@@ -42,6 +42,7 @@ stdenv.mkDerivation (finalAttrs: {
     gtk-doc
     docbook-xsl-nons
     docbook_xml_dtd_43
+    wayland-scanner
     vala
     wayland-scanner
   ];