spec: use pkgconf to find out krb5 version
    
    In Fedora 33 RPM migrated to use SQLITE as its database format. When
    COPR builders run on Fedora 33 and build a package for older Fedora
    version that uses RPM with BDB backend, RPM inside the build environment
    will not be able to open its own database (SQLITE).
    
    Replace use of RPM to discover krb5 version by use of pkgconf which
    provides the same output but doesn't need to look into RPM database.
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>