MAINTAINERS: add additional mmap-related files to mmap section

MAINTAINERS: add additional mmap-related files to mmap section

msync and nommu are directly related to memory mapping, mincore is less so
but all are roughly speaking operating on virtual memory mappings from the
point of view of the user so this seems the most appropriate place for
them.

Link: https://lkml.kernel.org/r/20250617144130.147847-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>