mvneta: Add missing if_vlan.h include.

drivers/net/ethernet/marvell/mvneta.c: In function 'mvneta_skb_tx_csum':
drivers/net/ethernet/marvell/mvneta.c:1374:3: error: implicit declaration of function 'vlan_get_protocol' [-Werror=implicit-function-declaration]
   __be16 l3_proto = vlan_get_protocol(skb);
   ^

Reporeted-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>