Fix @Qt5X11Extras_FOUND@ condition in cmake config file
Can't use @Qt5X11Extras_FOUND@ because it'll be undefined and evaluated to an empty string on non-X11 builds, and when building against Qt6; also can't use 'TARGET Qt5::X11Extras' because it my be undefined at the time this file is used, instead use Qt6::Gui which will be set by the find_dependency call some lines above.
For more details see: kguiaddons!37 (merged)
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