plasma54.kscreen: propagate libkscreen dependency

libkscreen provides a D-Bus service.

pkgs/desktops/plasma-5.4/kscreen.nix

···
     kconfig kconfigwidgets kdbusaddons kwidgetsaddons kxmlgui
   ];
   propagatedBuildInputs = [ kglobalaccel ki18n libkscreen qtdeclarative ];
+  propagatedUserEnvPkgs = [
+    libkscreen  # D-Bus service
+  ];
   postInstall = ''
     wrapQtProgram "$out/bin/kscreen-console"
   '';