MAINTAINERS: update the LSM and SELinux subsystems

The SELinux code, security/selinux/, already has a MAINTAINERS entry
so exclude it from the security subsystem entry in an effort to better
reflect current practices.
Signed-off-by: Paul Moore <paul@paul-moore.com>

MAINTAINERS

@@ -12769,6 +12769,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git
 W:	http://kernsec.org/
 S:	Supported
 F:	security/
+X:	security/selinux/
 
 SELINUX SECURITY MODULE
 M:	Paul Moore <paul@paul-moore.com>