pylibmount: properly mark initialization function

pylibmount: properly mark initialization function

The module initialization function is supposed to only public function
in a module.
Newer versions of meson use -fvisibility=hidden and expected this
function to be marked with PyMODINIT_FUNC [0].

As this does not hurt on autotools either, let's use it everywhere.

[0] https://mesonbuild.com/Release-notes-for-0-63-0.html#python-extension-modules-now-build-with-hidden-visibility