libstdbuf: Move to the utilities package

This is a niche tool which is only useful in a small number of
specific situations. It's very small (8kB), and the executable
component /usr/bin/stdbuf is already in -utilities, so move the
shlib to -utilities as well.

MFC after: 1 day
Reviewed by: bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52788