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

Reported by Andrew Walrond <andrew@walrond.org>

Changeset c19e654ddbe3831252f61e76a74d661e1a755530
("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>
diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index 12e9a29..6a9bb97 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -89,7 +89,6 @@
 header-y += if_slip.h
 header-y += if_strip.h
 header-y += if_tun.h
-header-y += if_tunnel.h
 header-y += in_route.h
 header-y += ioctl.h
 header-y += ip6_tunnel.h
@@ -235,6 +234,7 @@
 unifdef-y += if_pppol2tp.h
 unifdef-y += if_pppox.h
 unifdef-y += if_tr.h
+unifdef-y += if_tunnel.h
 unifdef-y += if_vlan.h
 unifdef-y += igmp.h
 unifdef-y += inet_diag.h