virtio: add missing include to virtio_net.h

virtio_net.h uses the macro ETH_ALEN which is defined in linux/if_ether.h.
Discovered when hacking on virtio-over-pci patches.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/virtio_net.h

@@ -4,6 +4,7 @@
  * compatible drivers/servers. */
 #include <linux/types.h>
 #include <linux/virtio_config.h>
+#include <linux/if_ether.h>
 
 /* The ID for virtio_net */
 #define VIRTIO_ID_NET	1