qt6.qtshadertools: add mingw support

Part of a larger effort to add mingw support for qtmultimedia.

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

@@ -1,8 +1,14 @@
 { qtModule
 , qtbase
+, stdenv
+, lib
+, pkgsBuildBuild
 }:
 
 qtModule {
   pname = "qtshadertools";
   propagatedBuildInputs = [ qtbase ];
+  cmakeFlags = lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
+    "-DQt6ShaderToolsTools_DIR=${pkgsBuildBuild.qt6.qtshadertools}/lib/cmake/Qt6ShaderToolsTools"
+  ];
 }