Add missing const after 6c4f95161d

I accidentally didn't include hunk in the committed patch.

Fixes:		6c4f95161d ("virtio: make the write_config buffer argument const")

sys/dev/virtio/mmio/virtio_mmio.c

@@ -741,7 +741,7 @@ vtmmio_write_dev_config(device_t dev, bus_size_t offset,
 {
 	struct vtmmio_softc *sc;
 	bus_size_t off;
-	uint8_t *s;
+	const uint8_t *s;
 	int size;
 
 	sc = device_get_softc(dev);