MAINTAINERS: Add myself as virtio-balloon co-maintainer

As suggested by Michael, let's add me as co-maintainer of virtio-balloon.
While at it, also add "include/linux/balloon_compaction.h" to the file
list.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200310115411.12760-1-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>