qt5.qttools: install macdeployqt on darwin

pkgs/development/libraries/qt-5/modules/qttools.nix

···
-{ qtModule, lib, qtbase }:
+{ qtModule, stdenv, lib, qtbase }:
 
 with lib;
 
···
     "bin/qhelpgenerator"
     "bin/qtplugininfo"
     "bin/qthelpconverter"
+  ] ++ optionals stdenv.isDarwin [
+    "bin/macdeployqt"
   ];
 
   setupHook = ../hooks/qttools-setup-hook.sh;