include/uapi/linux/virtio_balloon.h: include linux/virtio_types.h

Fixes userspace compilation error:

error: unknown type name ‘__virtio16’
  __virtio16 tag;

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>