net-tools: fix building with linux-4.8

net-tools: fix building with linux-4.8

Fix a build error when using the linux-4.8 headers that results in:

In file included from
.../sysroots/qemuarm64/usr/include/linux/if_tunnel.h:6:0,
                  from iptunnel.c:39:
.../qemuarm64/usr/include/linux/ip.h:85:8: error: redefinition of
'struct iphdr'
  struct iphdr {
         ^~~~~
In file included from iptunnel.c:29:0:
.../qemuarm64/usr/include/netinet/ip.h:44:8: note: originally defined here
  struct iphdr
         ^~~~~
(From OE-Core rev: 94d15885c516e3bfee4fb68dfb568f4da6904052)

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>