util/authtok: set destructor in sss_authtok_new()
sss_authtok_new() is the only legal way to create instance
of sss_auth_token. Setting destructor in this factory function allows
for secure erasure of sensitive data.
Relates: https://pagure.io/SSSD/sssd/issue/3956
Reviewed-by: Sumit Bose <sbose@redhat.com>