posture: add get serial stub for all platforms

Updates #5902

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

cmd/tailscaled/depaware.txt

@@ -292,6 +292,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/wsconn                                     from tailscale.com/control/controlhttp+
         tailscale.com/paths                                          from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
+        tailscale.com/posture                                        from tailscale.com/ipn/ipnlocal
         tailscale.com/proxymap                                       from tailscale.com/tsd+
         tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
         tailscale.com/smallzstd                                      from tailscale.com/control/controlclient+

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")
+}