xfrm: Make function xfrmi_get_link_net() static

Fixes the following sparse warning:

net/xfrm/xfrm_interface.c:745:12: warning:
 symbol 'xfrmi_get_link_net' was not declared. Should it be static?

Fixes: f203b76d ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>