net: Fix userland breakage wrt. linux/if_tunnel.h

Reported by Andrew Walrond <andrew@walrond.org>

Changeset c19e654d


("gre: Add netlink interface") added an include
of linux/ip.h to linux/if_tunnel.h

We can't really let that get exposed to userspace
because this conflicts with types defined in netinet/ip.h
which userland is almost certainly going to have included
either explicitly or implicitly.

So guard this include with a __KERNEL__ ifdef.

Signed-off-by: David S. Miller <davem@davemloft.net>