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

MAINTAINERS

@@ -18096,6 +18096,7 @@ M:	Joonsoo Kim <iamjoonsoo.kim@lge.com>
 M:	Andrew Morton <akpm@linux-foundation.org>
 M:	Vlastimil Babka <vbabka@suse.cz>
 R:	Roman Gushchin <roman.gushchin@linux.dev>
+R:	Hyeonggon Yoo <42.hyeyoo@gmail.com>
 L:	linux-mm@kvack.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git