Remove static on convert_bits

[CarloMara]

Dear all,

I'm working on a C implementation of age and I need access to the internal convert_bits function which is declared static. This pr enables that.

Happy new year, Carlo

[sipa]

I think you shouldn't use segwit_addr.c directly as it's specific for segwit addresses. Feel free to copy the convert_bits function though; it's trivial.