posture: add get serial stub for all platforms

Updates #5902

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

posture/serialnumber_stub.go

@@ -0,0 +1,11 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package posture
+
+import "errors"
+
+// GetSerialNumber returns client machine serial number(s).
+func GetSerialNumbers() ([]string, error) {
+	return nil, errors.New("not implemented")
+}