React: Include required render() function in base Component class.

The new TSX mode in the TypeScript compiler has the requirements that
the element class constructor produces an element instance type that is
assignable to the type JSX.ElementClass.  To meet this Component<P, S>
must declare `render(): JSX.Element`.  Derived classes can be more
specific.

This should resolve Microsoft/TypeScript#3928.

react/react-global.d.ts

@@ -136,6 +136,7 @@ declare module React {
         setState(f: (prevState: S, props: P) => S, callback?: () => any): void;
         setState(state: S, callback?: () => any): void;
         forceUpdate(): void;
+        render(): JSX.Element;
         props: P;
         state: S;
         context: any;

react/react.d.ts

@@ -136,6 +136,7 @@ declare module __React {
         setState(f: (prevState: S, props: P) => S, callback?: () => any): void;
         setState(state: S, callback?: () => any): void;
         forceUpdate(): void;
+        render(): JSX.Element;
         props: P;
         state: S;
         context: any;