If both audit and seccomp filter support are disabled, 'ret' is marked
as unused.

If just seccomp filter support is disabled, data and skip are considered
unused.

This change fixes those build warnings.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NWill Drewry <wad@chromium.org>
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>