CIFS ACL support needs CONFIG_KEYS, so depend on it

Build fails if CONFIG_KEYS is not selected.

Signed-off-by: Darren Salt <linux@youmustbejoking.demon.co.uk>
Reviewed-by: Shirish Pargaonkar <shirishp@us.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>