MAINTAINERS: add hugetlb_cgroup.c to hugetlb section

This file is clearly specific to hugetlb so this seems the most
appropriate place for it.

Link: https://lkml.kernel.org/r/20250617171538.178042-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ljskernel

MAINTAINERS

@@ -11169,6 +11169,7 @@ F:	fs/hugetlbfs/
 F:	include/linux/hugetlb.h
 F:	include/trace/events/hugetlbfs.h
 F:	mm/hugetlb.c
+F:	mm/hugetlb_cgroup.c
 F:	mm/hugetlb_cma.c
 F:	mm/hugetlb_cma.h
 F:	mm/hugetlb_vmemmap.c