Adding FB OSS header to GA analytics flow type (#3175)

Author: rachelnabors

flow-typed/gatsby-plugin-google-analytics.js

@@ -1,4 +1,6 @@
-// import {trackCustomEvent} from 'gatsby-plugin-google-analytics';
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ */
 
 declare module 'gatsby-plugin-google-analytics' {
   declare module.exports: {