MAINTAINERS: add entry for shrinker

Since the shrinker-related code has been moved to a separate shrinker.c
file, it's time to add a MAINTAINERS entry for it.

Dave, Roman, Muchun and I have all worked on shrinker (development,
review, etc) in the past period of time, and all of us are willing to
continue working on shrinker in the future, so I'd like to add all of us
as maintainer/reviewer.

Link: https://lkml.kernel.org/r/20240111075219.34221-1-zhengqi.arch@bytedance.com


Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>