mm: no need to export mm_kobj

There are no modules using mm_kobj, so do not export it.

Link: https://lkml.kernel.org/r/2023080436-algebra-cabana-417d@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>