build: fix build for Debian 9 and Debian 10

Type: fix

Change-Id: Ic07d0ae313b32e420ba93693cb75960a86f752a9
Signed-off-by: Benoît Ganne <bganne@cisco.com>
diff --git a/src/vnet/devices/virtio/virtio.h b/src/vnet/devices/virtio/virtio.h
index f1eaa07..b00e1ec 100644
--- a/src/vnet/devices/virtio/virtio.h
+++ b/src/vnet/devices/virtio/virtio.h
@@ -129,7 +129,15 @@
 
   /* error */
   clib_error_t *error;
-  u8 mac_addr[6];
+  union
+  {
+    struct
+    {
+      u32 mac_addr32;
+      u16 mac_addr16;
+    };
+    u8 mac_addr[6];
+  };
   union
   {
     struct			/* tun/tap interface */