qt_common: consistently ifdef QPlatform after cbd79df2

src/yuzu/qt_common.cpp:45:33: error: member access into incomplete type 'QPlatformNativeInterface'
    wsi.display_connection = pni->nativeResourceForWindow("display", window);
                                ^
/usr/include/qt6/QtGui/qguiapplication.h:20:7: note: forward declaration of 'QPlatformNativeInterface'
class QPlatformNativeInterface;
      ^
src/yuzu/qt_common.cpp:47:42: error: member access into incomplete type 'QPlatformNativeInterface'
        wsi.render_surface = window ? pni->nativeResourceForWindow("surface", window) : nullptr;
                                         ^
/usr/include/qt6/QtGui/qguiapplication.h:20:7: note: forward declaration of 'QPlatformNativeInterface'
class QPlatformNativeInterface;
      ^