MAINTAINERS: update the LSM maintainer info

After many years of shepherding the LSM layer, and Linux kernel
access control in general, James has decided to take a small step
away from his technical role and has asked me to assume the
day-to-day maintenance tasks for the LSM.  This patch makes the
necessary updates to the MAINTAINERS file.

Thanks for all you patience and stewardship over the years James,
I promise to do my best not to screw it all up :)

Cc: linux-security-module@vger.kernel.org
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>