Enable completion for flux tree cmd

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/tree_kustomization.go

@@ -51,6 +51,7 @@ var treeKsCmd = &cobra.Command{
   # Print the Flux resources managed by the root Kustomization
   flux tree kustomization flux-system --compact`,
 	RunE:              treeKsCmdRun,
+	ValidArgsFunction: resourceNamesCompletionFunc(kustomizev1.GroupVersion.WithKind(kustomizev1.KustomizationKind)),
 }
 
 type TreeKsFlags struct {