qtutilities: Add Darwin to supported platforms

pkgs/development/libraries/qtutilities/default.nix

···
     description = "Common Qt related C++ classes and routines used by @Martchus' applications such as dialogs, widgets and models Topics";
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ doronbehar ];
-    platforms   = platforms.linux;
+    platforms   = platforms.linux ++ platforms.darwin;
   };
 }