Enable pod security warnings for flux-system

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

manifests/install/namespace.yaml

@@ -2,3 +2,6 @@ apiVersion: v1
 kind: Namespace
 metadata:
   name: flux-system
+  labels:
+    pod-security.kubernetes.io/warn: restricted
+    pod-security.kubernetes.io/warn-version: latest

pkg/manifestgen/install/templates.go

@@ -165,6 +165,9 @@ apiVersion: v1
 kind: Namespace
 metadata:
   name: {{.Namespace}}
+  labels:
+    pod-security.kubernetes.io/warn: restricted
+    pod-security.kubernetes.io/warn-version: latest
 `
 
 func execTemplate(obj interface{}, tmpl, filename string) error {