security: Wrap SMACK and SELINUX audit data structs in ifdefs

Wrapped the smack_audit_data and selinux_audit_data
structs in include/linux/lsm_audit.h in ifdefs so that the
union will always be the correct size.

Signed-off-by: Thomas Liu <tliu@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>