net-tools: add a patch to avoid struct redefinition

This fixes a struct redefinition problem like this one:

================================
In file included from /usr/include/linux/if_tunnel.h:6:0,
from iptunnel.c:34:
/usr/include/linux/ip.h:85:8: error: redefinition of 'struct iphdr'
struct iphdr {
^
In file included from iptunnel.c:29:0:
/usr/include/netinet/ip.h:45:8: note: originally defined here
struct iphdr
^
================================

iptunnel.c includes netinet/ip.h which contains a definition of the
iphdr struct.

iptunnel.c also includes linux/if_tunnel.h which includes linux/ip.h
which contains a definition of the iphdr struct.

So, both netinet/ip.h and linux/ip.h define the iphdr struct, and both
of them have been included directly or indirectly by iptunnel.c. Because
of that the compilation fails due to a struct redefinition.

The problem can be solved by just not including netinet/ip.h.

The patch has been sent upstream as a merge request:
  https://sourceforge.net/p/net-tools/code/merge-requests/3/

Fixes:
  http://autobuild.buildroot.net/results/dce/dce499da84b2a41bab946d5109a283ccb85c8b81/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>