pre_user_query.php

<?php

if ( '' != $query->query_vars['search'] ) {
    $meta_term = str_replace( '*', '', $query->query_vars['search'] );
    $query->query_fields = 'DISTINCT SQL_CALC_FOUND_ROWS wp_users.ID';
    $query->query_from   = "FROM $wpdb->users LEFT JOIN $wpdb->usermeta ON $wpdb->usermeta.user_id = $wpdb->users.ID";
    $query->query_where .= " OR display_name LIKE '%$meta_term%' OR ($wpdb->usermeta.meta_key = 'first_name' AND $wpdb->usermeta.meta_value = '{$meta_term}') OR ($wpdb->usermeta.meta_key = 'last_name' AND $wpdb->usermeta.meta_value = '{$meta_term}')";
}