Replace your code with this. This warning is because PHP4 style constructors are depreciated since WordPress 4.3
/**
* @Agents(s) list widget Class
*/
if ( ! class_exists( 'cs_agentlist' ) ) {
class cs_agentlist extends WP_Widget {
/**
* Outputs the content of the widget
* @param array $args
* @param array $instance
*/
/**
* @init User list Module
*/
public function __construct() {
$widget_ops = array('classname' => 'widget_agents', 'description' => 'Select user to show in widget.');
parent::__construct('cs_agentlist', 'CS : Agents', $widget_ops);
}