p11-kit: add serial number in DER format
    
    This causes Firefox to report our CA certificate as not-trustworthy.
    We were previously doing this correctly, however it slipped as an
    error due to certificate refactoring.
    
    https://pagure.io/freeipa/issue/7210
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>