Export RouterOnContext from "react-router" module.

react-router/react-router.d.ts

@@ -451,6 +451,7 @@ declare module "react-router" {
     export type RouterListener = ReactRouter.RouterListener
     export type RouterState = ReactRouter.RouterState
     export type HistoryBase = ReactRouter.HistoryBase
+    export type RouterOnContext = ReactRouter.RouterOnContext
 
     export {
         Router,