unbound: Fix config file path

Commit 1838dec31895fd4752fa8631322ab93be0705a66 changed the
config file path to /usr/local/etc/unbound/unbound.conf which
is wrong for unbound in base.

Reported by: Ihor Antonov <ihor_AT_antonovs_family>
Reviewed by: zlei
Approved by: zlei
Differential Revision: https://reviews.freebsd.org/D38106

Fixes: 1838dec31895fd4752fa8631322ab93be0705a66

(cherry picked from commit 1d577bedbae80ced38ccb834e6835e5fd98bc411)