Add login/logout links to WP menus

add_login_logout_link.php

add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2); 
function add_login_logout_link($items, $args) {         
  ob_start();         
  wp_loginout('index.php');         
  $loginoutlink = ob_get_contents();        
  ob_end_clean();         
  $items .= '<li>'. $loginoutlink .'</li>';     
  return $items; 
}