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>