Fix cloneElement Typescript definition (#1197)

* Fix cloneElement Typescript definition

The cloneElement function accepts an optional third argument to override the cloned element's children.

src/preact.d.ts

@@ -114,7 +114,7 @@ declare namespace preact {
 
 	function render(node: ComponentChild, parent: Element | Document | ShadowRoot | DocumentFragment, mergeWith?: Element): Element;
 	function rerender(): void;
-	function cloneElement(element: JSX.Element, props: any): JSX.Element;
+	function cloneElement(element: JSX.Element, props: any, ...children: ComponentChildren[]): JSX.Element;
 
 	var options: {
 		syncComponentUpdates?: boolean;