virtio: Add module.h to drivers/virtio users.

Up to now, the module.h header was as hard to keep out as
sunlight.  But we are cleaning that up.  Fix the virtio users
who simply expect module.h to be there in every C file.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4acf888..c7a2c20 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -21,6 +21,7 @@
 #include <linux/virtio_config.h>
 #include <linux/device.h>
 #include <linux/slab.h>
+#include <linux/module.h>
 
 /* virtio guest is communicating with a virtual "device" that actually runs on
  * a host processor.  Memory barriers are used to control SMP effects. */