MAINTAINERS: Add security subsystem maintainer

Add myself as overall maintainer of the security subsystem (generally,
components under the top-level security directory).  This addresses
the lack of an official maintainer for the increasing number of
security projects being incorporated into the kernel.

Signed-off-by: James Morris <jmorris@namei.org>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>