posture: add get serial support for Windows/Linux

This commit adds support for getting serial numbers from SMBIOS
on Windows/Linux (and BSD) using go-smbios.

Updates #5902

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

posture/serialnumber_stub.go

@@ -1,11 +1,24 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+// darwin: not implemented
+// andoird: not implemented
+// js: not implemented
+// plan9: not implemented
+// solaris: currently unsupported by go-smbios:
+// https://github.com/digitalocean/go-smbios/pull/21
+
+//go:build darwin || android || js || plan9 || solaris
+
 package posture
 
-import "errors"
+import (
+	"errors"
+
+	"tailscale.com/types/logger"
+)
 
 // GetSerialNumber returns client machine serial number(s).
-func GetSerialNumbers() ([]string, error) {
+func GetSerialNumbers(_ logger.Logf) ([]string, error) {
 	return nil, errors.New("not implemented")
 }