netcat-openbsd: fix w/musl by providing b64_ntop

It seems many projects have copies of this code--
and AFAICT all are derived from ISC's BIND[2]
although it does not appear present in recent versions.

This is a curious function as it is not documented
nor part of any standard[1].

[1] https://www.lemoda.net/unix/base64/index.html
[2] for example, glibc's copy:
    https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/README;h=514e9bb617e710f16126c1474561965a2b35653d;hb=HEAD#l104