include: fix build with kernel headers before 4.2

include: fix build with kernel headers before 4.2

Commit 672accf1530 (include: update kernel netfilter header files)
updated linux/netfilter.h and brought with it the update from kernel
commit a263653ed798 (netfilter: don't pull include/linux/netfilter.h
from netns headers). This triggers conflict of headers that is fixed in
kernel commit 279c6c7fa64f (api: fix compatibility of linux/in.h with
netinet/in.h) included in kernel version 4.2. For earlier kernel headers
we need a workaround that prevents the headers conflict.

Fixes the following build failure:

In file included from .../sysroot/usr/include/netinet/ip.h:25:0,
                 from ../include/libiptc/ipt_kernel_headers.h:8,
                 from ../include/libiptc/libiptc.h:6,
                 from libip4tc.c:29:
.../sysroot/usr/include/linux/in.h:26:3: error: redeclaration of enumerator ‘IPPROTO_IP’
   IPPROTO_IP = 0,  /* Dummy protocol for TCP  */
   ^
.../sysroot/usr/include/netinet/in.h:33:5: note: previous definition of ‘IPPROTO_IP’ was here
     IPPROTO_IP = 0,    /* Dummy protocol for TCP.  */
     ^~~~~~~~~~

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Florian Westphal <fw@strlen.de>