enable fvisibility=hidden for our private library

enable fvisibility=hidden for our private library

While it is a huge undertaking to enable it for our public libraries as
basically everything we exported so far could be seen as public
interface our private library is new and under our full control, so we
can do whatever we like with it. The benefits are not that big in return
of course, but it reduces the size a bit, so thats great nontheless.

Git-Dch: ignore