net: xfrm: unexport __init-annotated xfrm4_protocol_init()

In the Linux kernel, the following vulnerability has been resolved:

net: xfrm: unexport __init-annotated xfrm4_protocol_init()

EXPORT_SYMBOL and __init is a bad combination because the .init.text
section is freed up after the initialization. Hence, modules cannot
use symbols annotated __init. The access to a freed symbol may end up
with kernel panic.

modpost used to detect it, but it has been broken for a decade.

Recently, I fixed modpost so it started to warn it again, then this
showed up in linux-next builds.

There are two ways to fix it:

- Remove __init
- Remove EXPORT_SYMBOL

I chose the latter for this case because the only in-tree call-site,
net/ipv4/xfrm4_policy.c is never compiled as modular.
(CONFIG_XFRM is boolean)