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/if_tunnel.h b/include/linux/if_tunnel.h
index aeab2cb..82c43624 100644
--- a/include/linux/if_tunnel.h
+++ b/include/linux/if_tunnel.h
@@ -2,7 +2,10 @@
 #define _IF_TUNNEL_H_
 
 #include <linux/types.h>
+
+#ifdef __KERNEL__
 #include <linux/ip.h>
+#endif
 
 #define SIOCGETTUNNEL   (SIOCDEVPRIVATE + 0)
 #define SIOCADDTUNNEL   (SIOCDEVPRIVATE + 1)