netdevice: Add missing IFF_PHONY_HEADROOM self-definition

This is harmless for now, but can be fatal for future refactors.

Fixes: 871b642ade ("netdev: introduce ndo_set_rx_headroom")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210218204908.5455-2-alobakin@pm.me

include/linux/netdevice.h

@@ -1577,6 +1577,7 @@ enum netdev_priv_flags {
 #define IFF_L3MDEV_SLAVE		IFF_L3MDEV_SLAVE
 #define IFF_TEAM			IFF_TEAM
 #define IFF_RXFH_CONFIGURED		IFF_RXFH_CONFIGURED
+#define IFF_PHONY_HEADROOM		IFF_PHONY_HEADROOM
 #define IFF_MACSEC			IFF_MACSEC
 #define IFF_NO_RX_HANDLER		IFF_NO_RX_HANDLER
 #define IFF_FAILOVER			IFF_FAILOVER