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

net/openvswitch/flow_netlink.c

@@ -338,7 +338,7 @@ size_t ovs_tun_key_attr_size(void)
 		+ nla_total_size(4);   /* OVS_TUNNEL_KEY_ATTR_ERSPAN_OPTS */
 }
 
-size_t ovs_nsh_key_attr_size(void)
+static size_t ovs_nsh_key_attr_size(void)
 {
 	/* Whenever adding new OVS_NSH_KEY_ FIELDS, we should consider
 	 * updating this function.