virtio: fix trivial build bug

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: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>