qt6.qtwayland: fix cross compilation

pkgs/development/libraries/qt-6/modules/qtwayland.nix

@@ -1,4 +1,6 @@
 {
+  pkgsBuildBuild,
+  stdenv,
   lib,
   qtModule,
   qtbase,
@@ -38,6 +40,10 @@ qtModule {
   buildInputs = [ libdrm ];
   nativeBuildInputs = [ pkg-config ];
 
+  cmakeFlags = lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
+    "-DQt6WaylandScannerTools_DIR=${pkgsBuildBuild.qt6.qtwayland}/lib/cmake/Qt6WaylandScannerTools"
+  ];
+
   meta = {
     platforms = lib.platforms.unix;
     badPlatforms = lib.platforms.darwin;