Taskbar: Include ScreenLayout.h from Services directory

While trying to include `Desktop.h` for the SDL2 port, compilation
failed on finding this include. Specify the full include path to make
it work.

Userland/Services/Taskbar/TaskbarWindow.h

@@ -12,7 +12,7 @@
 #include <LibGUI/Widget.h>
 #include <LibGUI/Window.h>
 #include <LibGfx/ShareableBitmap.h>
-#include <WindowServer/ScreenLayout.h>
+#include <Services/WindowServer/ScreenLayout.h>
 
 class TaskbarWindow final : public GUI::Window {
  C_OBJECT(TaskbarWindow);