openvswitch: Make local function ovs_nsh_key_attr_size() static

Fixes the following sparse warnings:

net/openvswitch/flow_netlink.c:340:8: warning:
 symbol 'ovs_nsh_key_attr_size' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>