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

MAINTAINERS

@@ -13622,6 +13622,7 @@ F:	include/linux/memory_hotplug.h
 F:	include/linux/mm.h
 F:	include/linux/mmzone.h
 F:	include/linux/pagewalk.h
+F:	include/linux/rmap.h
 F:	include/trace/events/ksm.h
 F:	mm/
 F:	tools/mm/