MAINTAINERS: add missing headers to mempory policy & migration section

These two files currently do not belong to any section.
The memory policy & migration section seems to be a good home for them!

Link: https://lkml.kernel.org/r/20250725175616.2397031-1-joshua.hahnjy@gmail.com
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: joshuahahn

MAINTAINERS

@@ -15849,7 +15849,9 @@ S:	Maintained
 W:	http://www.linux-mm.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
 F:	include/linux/mempolicy.h
+F:	include/uapi/linux/mempolicy.h
 F:	include/linux/migrate.h
+F:	include/linux/migrate_mode.h
 F:	mm/mempolicy.c
 F:	mm/migrate.c
 F:	mm/migrate_device.c