MAINTAINERS: add missing mm/workingset.c file to mm reclaim section

The working set logic belongs very much to the reclaim section and is
otherwise not assigned to any other MAINTAINERS section so add it here.

Link: https://lkml.kernel.org/r/20250616201643.561626-1-lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>