base64_* -> kr_base64_*

GnuTLS 3.3 contained these as public symbols.  This doesn't seem
important, but why not use more collision-resistant names anyway?
Details/discussion: https://github.com/CZ-NIC/knot/pull/14
Basically noone should notice, so I didn't include any NEWS entry.
25 jobs for base64-namespace
in 43 minutes and 50 seconds and was queued for 2 seconds