sit: make function ipip6_valid_ip_proto() static

Fixes the following sparse warning:

net/ipv6/sit.c:1129:6: warning:
 symbol 'ipip6_valid_ip_proto' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>