NEGCACHE: Always add "root" to the negative cache
    
    The current code only adds "root" to the negative cache in case there's
    any other user or group set up in to be added.
    
    As SSSD doesn't handle "root", it should *always* be added to the
    negative cache.
    
    Related: https://pagure.io/SSSD/sssd/issue/3460
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 1e7b7da3aa56060c26f8ba1c08318cdee77753ea)
    (cherry picked from commit 2ad3a17831f33629faaa334804636ef367e2fafb)