xfrm: Make function xfrm_dev_register static

Fixes the following sparse warning:

net/xfrm/xfrm_device.c:141:5: warning:
 symbol 'xfrm_dev_register' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>