ipa-kdb: fix gcc complaints in kdb tests
    
    We use string_to_sid() from internal Samba libraries, so we have to link
    to it properly.
    
    In addition, size_t is (long unsigned int), just cast to (int) in
    asprintf.
    
    Fixes: https://pagure.io/freeipa/issue/8585
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>