unbound: Move config.h to lib directory.

It was originally in contrib, and moved to usr.sbin in 6692aa840c1f; I always thought lib would make more sense but never got around to moving it.

Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D41504

(cherry picked from commit 2ba1d4970a06a1660b46f6fd99351d154b295683)