FindWaylandScanner: Add flag to use private-code
This setting hides the generated client code from export in a shared library, which is recommended by wayland-scanner's own help files.
Few points that I'm unsure on: Do I need to add a matching PUBLIC keyword to explicitly request the default of public-code? I don't think it's needed but it it would match things like target_include_directories and friends.
I'm not sure if the help text is quite right, rewording welcome.
It's my first time contributing for KDE, so please be gentle. :)