include: extend the headers conflict workaround to in6.h

include: extend the headers conflict workaround to in6.h

Commit 8d9d7e4b9ef ("include: fix build with kernel headers before 4.2")
introduced a kernel/user headers conflict workaround that allows build
of iptables with kernel headers older than 4.2. This minor extension
allows build with kernel headers older than 3.12, which is the version
that introduced explicit IP headers synchronization.

Fixes: 8d9d7e4b9ef4 ("include: fix build with kernel headers before 4.2")
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>