iptunnel: fix building w/older linux headers
While linux-4.8+ headers work, older ones are missing includes. Dropping netinet/ip.h uncovered that mess, so add linux/ip.h.
URL: https://bugs.gentoo.org/599542 Reported-by: Conrad Kostecki <ck@...>
Authored by: Mike Frysinger 2016-11-15
Parent: [45d573]
Child: [aab927]