virtio.h uses struct module, add a forward declaration to
make the header self-contained.
Message-ID: <9171b5cac60793eb59ab044c96ee038bf1363bee.1764873799.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>