unix: add nl80211 generic netlink constants for Linux

Change-Id: Ic39f350bd0b4e3942ba576ee12b44b182178527c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/388935
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/linux/mksysnum.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build ignore
 // +build ignore
 
 package main