Add Matt to MAINTAINERS as a SLAB allocator maintainer

Matt is already the maintainer of SLOB which is one of the "SLAB" allocators in
the kernel so add him to MAINTAINERS.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>