fixed app-layer generator, re-enabled struct2interface in make… (#14260)

* fixed app-layer generator, re-enabled struct2interface in makefile

Makefile

@@ -200,9 +200,8 @@ endif
 endif
 
 app-layers: ## Extract interface from App struct
-    # The following commented commands can be used to re-generate the AppIface from the App struct
-	#env GO111MODULE=off $(GO) get gopkg.in/reflog/struct2interface.v0
-	#$(GOBIN)/struct2interface.v0 -f "app" -o "app/app_iface.go" -p "app" -s "App" -i "AppIface" -t ./app/layer_generators/app_iface.go.tmpl
+	env GO111MODULE=off $(GO) get gopkg.in/reflog/struct2interface.v0
+	$(GOBIN)/struct2interface.v0 -f "app" -o "app/app_iface.go" -p "app" -s "App" -i "AppIface" -t ./app/layer_generators/app_iface.go.tmpl
 	$(GO) run ./app/layer_generators -in ./app/app_iface.go -out ./app/opentracing_layer.go -template ./app/layer_generators/opentracing_layer.go.tmpl
 
 i18n-extract: ## Extract strings for translation from the source code