fix build bug:

  drivers/virtio/virtio_balloon.c: In function 'fill_balloon':
  drivers/virtio/virtio_balloon.c:98: error: implicit declaration of function 'msleep'
Acked-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NIngo Molnar <mingo@elte.hu>