MAINTAINERS: add an entry for the lockdown LSM

While lockdown has been present in the kernel for a while, it is
missing a MAINTAINERS entry for some reason.

Signed-off-by: Matthew Garrett <[e-mail]>
Signed-off-by: Paul Moore <[e-mail]>

MAINTAINERS

···
 F:	Documentation/admin-guide/LSM/LoadPin.rst
 F:	security/loadpin/
 
+LOCKDOWN SECURITY MODULE
+L:	[e-mail]
+S:	Odd Fixes
+T:	git https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/lsm.git
+F:	security/lockdown/
+
 LOCKING PRIMITIVES
 M:	Peter Zijlstra <[e-mail]>
 M:	Ingo Molnar <[e-mail]>