MAINTAINERS: add myself as reviewer for slab

Recently I was involved in slab subsystem (reviewing struct slab,
SLUB debugfs and etc). I would like to help maintainers and people
working on slab allocators by reviewing and testing their work.

Let me be Cc'd on patches related to slab.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20220507073506.241963-1-42.hyeyoo@gmail.com