Fixes the following sparse warning:

net/8021q/vlan_core.c:168:6: warning:
 symbol 'vlan_hw_filter_capable' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>