Part of the previous commit accidentally left out

I've accidentally committed a partial patch. This commit
adds missing parts of the patch to the repo.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Set default visibility of symbols to "hidden"

This causes symbols not to be exported from the library
unless they are explicitly marked by NN_EXPORT.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>