virtio: make it self-contained

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>

Author: mstsirkin

include/linux/virtio.h

@@ -13,6 +13,8 @@
 #include <linux/completion.h>
 #include <linux/virtio_features.h>
 
+struct module;
+
 /**
  * struct virtqueue - a queue to register buffers for sending or receiving.
  * @list: the chain of virtqueues for this device