net: Fix docbook warning for IFF_VRF_MASTER enum

kbuild test robot reported:
tree:   git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git master
head:   d52736e2
commit: 4e3c8992 [751/762] net: Introduce VRF related flags and helpers
reproduce: make htmldocs

>> Warning(include/linux/netdevice.h:1293): Enum value 'IFF_VRF_MASTER' not described in enum 'netdev_priv_flags'
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1262,6 +1262,7 @@ struct net_device_ops {
  * @IFF_LIVE_ADDR_CHANGE: device supports hardware address
  *	change when it's running
  * @IFF_MACVLAN: Macvlan device
+ * @IFF_VRF_MASTER: device is a VRF master
  * @IFF_NO_QUEUE: device can run without qdisc attached
  */
 enum netdev_priv_flags {