Add href property to DropdownItem (#16711)

The `DropdownItem` component was recently modified to accept an `href` prop (reactstrap/reactstrap#367).

/cc @alihammad, @mfal, @danilobjr

types/reactstrap/lib/DropdownItem.d.ts

@@ -5,7 +5,8 @@ interface Props {
   header?: boolean;
   onClick?: (event: React.MouseEvent<any>) => void;
   className?: string;
+  href?: string;
 }
 
 declare var DropdownItem: React.StatelessComponent<Props>;
-export default DropdownItem;
+export default DropdownItem;