Fatal error: Uncaught Error: Call to undefined function do_action_ref_array() when using wp_signon function

You should BIND the codes into action (you can use plugins_loaded for earliest stage):

add_action('plugins_loaded', 'my_func');
function my_func(){

  // ======== HERE YOUR CODES =========//

}

p.s. you have to include the file correctly!

include_once(ABSPATH.'wp-includes/user.php');