Guest Authors: Add labels property

Dynamically created class properties are deprecated in PHP 8.2, so this change explicitly defines the property in the class.

It needs to be public, as it is used by the list table class when defining the search box label.

php/class-coauthors-guest-authors.php

@@ -8,7 +8,7 @@
 
 class CoAuthors_Guest_Authors {
 
-
+	public $labels;
 	var $post_type              = 'guest-author';
 	var $parent_page            = 'users.php';
 	var $list_guest_authors_cap = 'list_users';