kdeFramework.kdewebkit: disable new designer plugin option

pkgs/development/libraries/kde-frameworks/kdewebkit.nix

···
   buildInputs = [ kconfig kcoreaddons kio kparts ];
   propagatedBuildInputs = [ qtwebkit ];
   outputs = [ "out" "dev" ];
+  cmakeFlags = [
+    "-DBUILD_DESIGNERPLUGIN=OFF"
+  ];
 }