KNewStuff - CMake targets hardcoding paths

[Ben Cooksley]

Hi all,

On Windows the Binary Factory has recently struck an issue with
KNewStuff where the paths supplied in the newstuffqmlplugin target it
installs are hardcoded.

This can be seen at the bottom of
https://binary-factory.kde.org/view/Windows%2032-bit/job/Peruse_Nightly_win32/12/consoleText

Could someone please take a look and change it so that this is
relative like all the other Frameworks (and other parts of KNewStuff)

Thanks,
Ben