MAINTAINERS: add missing percpu-internal.h file to per-cpu section

This file seems to most appropriately belong to the PER-CPU MEMORY
ALLOCATOR section, so place it there.

Link: https://lkml.kernel.org/r/20250722171023.139777-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nico Pache <npache@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

MAINTAINERS

@@ -19459,6 +19459,7 @@ F:	arch/*/include/asm/percpu.h
 F:	include/linux/percpu*.h
 F:	lib/percpu*.c
 F:	mm/percpu*.c
+F:	mm/percpu-internal.h
 
 PER-TASK DELAY ACCOUNTING
 M:	Balbir Singh <bsingharora@gmail.com>