tempfork/wireguard-windows: remove unnecessary build tag

The _windows.go suffix suffices.
This allows go:generate to run without creating a diff.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tempfork/wireguard-windows/firewall/zsyscall_windows.go

@@ -1,5 +1,3 @@
-// +build windows
-
 // Code generated by 'go generate'; DO NOT EDIT.
 
 package firewall