igb: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/ethernet/intel/igb/igb_ethtool.c:2707:5: warning:
 symbol 'igb_rxnfc_write_vlan_prio_filter' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_ethtool.c

@@ -2704,7 +2704,7 @@ static int igb_rxnfc_write_etype_filter(struct igb_adapter *adapter,
 	return 0;
 }
 
-int igb_rxnfc_write_vlan_prio_filter(struct igb_adapter *adapter,
+static int igb_rxnfc_write_vlan_prio_filter(struct igb_adapter *adapter,
 					    struct igb_nfc_filter *input)
 {
 	struct e1000_hw *hw = &adapter->hw;