MAINTAINERS: add entry for shrinker

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/[email protected]
Signed-off-by: Qi Zheng <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>