postgresql: Set instance label for Argo CD

Argo CD wants every resource managed by an application to have that
application's name as the value of the `app.kubernetes.io/instance`
label.

postgresql/kustomization.yaml

@@ -1,6 +1,10 @@
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 
+labels:
+- pairs:
+    app.kubernetes.io/instance: postgresql
+
 namespace: postgresql
 
 resources: