LinuxKPI: add nexthdr definitions for IPv6

Add the nexthdr definitions for IPv6 which are used by wireless
drivers and were previously placed in an 80211 header file by
accident.

Obtained from: bz_iwlwifi
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b9d984e2c5dcef277c49a576ccefada6519d9b53)