MAINTAINERS: make Gregory a co-maintainer of MEMORY MANAGEMENT - NUMA PLACEMENT

Gregory is extremely familiar with NUMA placement handling and volunteer
to help co-maintain the numa placement bits.  So add him as a
co-maintainer.

Link: https://lore.kernel.org/20260918-maintainers-mempolicy-v1-3-9a94cac6d135@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Gregory Price (Meta) <gourry@gourry.net>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: SJ Park <sj@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Suren Baghdasaryan <surenb@google.com>