ipv6: sr: fix non static symbol warnings

Fixes the following sparse warnings:

net/ipv6/seg6_iptunnel.c:58:5: warning:
 symbol 'nla_put_srh' was not declared. Should it be static?
net/ipv6/seg6_iptunnel.c:238:5: warning:
 symbol 'seg6_input' was not declared. Should it be static?
net/ipv6/seg6_iptunnel.c:254:5: warning:
 symbol 'seg6_output' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>