adding source arn to lambda permission

main.tf

@@ -43,5 +43,6 @@ resource "aws_lambda_permission" "allow_cloudwatch" {
     statement_id = "AllowExecutionFromCloudWatch"
     action = "lambda:InvokeFunction"
     function_name = "${aws_lambda_function.packer_janitor.arn}"
+    source_arn = "${aws_cloudwatch_event_rule.packer_janitor.arn}"
     principal = "events.amazonaws.com"
-}
\ No newline at end of file
+}