Hide private symbols not hidden by default visibility about libarcus

[Ghostkeeper]

I'm not sure why the Socket::Private symbols were still in the build. Indeed I've seen crashes in the past in Socket::Private from CuraEngine so it must've been exposed somewhere. I've tried your suggestion of explicitly hiding it and everything still works fine.

We've not really put a lot of effort into giving this a proper stable ABI. I think the PIMPL pattern was implemented mostly as an experiment. This library doesn't change very often after all. Since this change works for you and doesn't hurt for the rest, I'll merge it.

[onitake]

Thanks for the quick response!
I know that your main focus has always been on the Python library, and libArcus was more of a glue component for CuraEngine.
But it's always better to be a bit mindful about such things.

With these fixes, the exported symbols from the library are vastly reduced: https://salsa.debian.org/3dprinting-team/libarcus/blob/master/debian/libarcus3.symbols
There's still a minor discrepancy between gcc8 and gcc9, but it's much less than it was before.