Link against Qt6::GuiPrivate to resolve qpa/qplatformnativeinterface.h: No such file or directory

[trstruth]

Trying to build without linking Qt6::GuiPrivate results in the following error:

  /home/trstruth/projects/dolphin/Source/Core/DolphinQt/MainWindow.cpp:34:10: fatal error:
  qpa/qplatformnativeinterface.h: No such file or directory
     34 | #include <qpa/qplatformnativeinterface.h>
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I was able to resolve this with the diff in the PR, but CMake emits this new warning:

  CMake Warning at /usr/lib/cmake/Qt6/QtPublicDependencyHelpers.cmake:335 (message):
    This project is using headers of the GuiPrivate module and will therefore
    be tied to this specific Qt module build version.  Running this project
    against other versions of the Qt modules may crash at any arbitrary point.
    This is not a bug, but a result of using Qt internals.  You have been
    warned!

    You can disable this warning by setting QT_NO_PRIVATE_MODULE_WARNING to ON.
  Call Stack (most recent call first):
    /usr/lib/cmake/Qt6GuiPrivate/Qt6GuiPrivateConfig.cmake:58
  (_qt_internal_show_private_module_warning)
    /usr/lib/cmake/Qt6/Qt6Config.cmake:247 (find_package)
    Source/Core/DolphinQt/CMakeLists.txt:17 (find_package)

I'm unable to evaluate if this is a problem for the maintainers, but just wanted to share this as it was preventing me from building from source on my machine

                  -`                     trstruth@venus
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