Security: remove security.h include from mm.h

Remove security.h include from mm.h, as it is only needed for a single
extern declaration, and pulls in all kinds of crud.

Fine-by-me: David Chinner <dgc@sgi.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>