Downgrade the minimal pkg-config version from 0.25 to 0.23.

This downgrade targets the Red Hat Enterprise Linux 6 distribution (and clones) as it ships only with pkg-config 0.23.
With this change we can use the same specfile to create libsodium binary RPMS for Fedora 18+ and EPEL6 [1]

/jpo

[1] - Add-on repository for RHEL 6 and clones (CentOS, Scientific Linux, Oracle Unbreakable Linux, ...)