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_test.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package posture
+
+import (
+	"testing"
+
+	"tailscale.com/types/logger"
+)
+
+func TestGetSerialNumber(t *testing.T) {
+	// ensure GetSerialNumbers is implemented
+	// or covered by a stub on a given platform.
+	_, _ = GetSerialNumbers(logger.Discard)
+}