spec file: bump minimum required pki-core version
    
    Require pki-core >= 10.2.6-19, which contains fixes for the following PKI
    tickets:
    * https://fedorahosted.org/pki/ticket/2022
    * https://fedorahosted.org/pki/ticket/2247
    * https://fedorahosted.org/pki/ticket/2255
    
    https://fedorahosted.org/freeipa/ticket/5602
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>