MAINTAINERS: add rmap.h to mm entry

Make it easier to figure out where to send patches for this file.

Link: https://lkml.kernel.org/r/efbc7689d35a48ff402644d696aa9a8d8bb6333a.1692877089.git.baruch@tkos.co.il


Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>