MAINTAINERS: add Jann Horn as rmap reviewer

Jann has been an excellent contributor in all areas of memory management,
and has demonstrated great expertise in the reverse mapping.

It's therefore appropriate for him to become a reviewer.

Link: https://lkml.kernel.org/r/20250908194959.820913-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: SeongJae Park <sj@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>