libnl build failure on some distros

[Cong Wang]

On Sat, Jun 28, 2014 at 12:49 AM, Thomas Graf <tgraf at suug.ch> wrote:
> We can include <netinet/in.h>, seems to be sufficient.

I thought ip6 tunnel has to use something in linux/in6.h,
it's definitely nice if we don't have to use it.

Thanks for the fix!