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: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>