LSM: SafeSetID: 'depend' on CONFIG_SECURITY

This patch changes the Kconfig file for the SafeSetID LSM to depend on
CONFIG_SECURITY as well as select CONFIG_SECURITYFS, since the policies
for the LSM are configured through writing to securityfs.

Signed-off-by: Micah Morton <mortonm@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>