Con not find class

Hi I am using wordpress for the first time. I am trying to incorperated the walker class into my project which is located at

D:\wamp\www\SgsOnline\wp-content\themes\storefront\inc\functions\walker.php

I have included my class in my functions as

require get_template_directory() . '/inc/functions/walker.php';

And I am trying to call it in my header like :

    function storefront_primary_navigation() {
                ?>
                <nav id="site-navigation" class="main-navigation " role="navigation" aria-label="<?php esc_html_e( 'Primary Navigation', 'storefront' ); ?>">
                <button class="menu-toggle" aria-controls="primary-navigation" aria-expanded="false"><?php echo esc_attr( apply_filters( 'storefront_menu_toggle_text', __( 'Navigation', 'storefront' ) ) ); ?></button>
                        <?php
                        wp_nav_menu(
                                array(
                                        'theme_location'        => 'primary',
                                        'container_class'       => 'primary-navigation',
                                        'menu_class'=>'nav navbar-nav navbar-left nav-tabs',
                                        'walker' => new Walker_Nav_Primary()
                                        )
                        );
                        ?>
                </nav><!-- #site-navigation -->
                <?php
        }

But I am getting a error :

Fatal error: Class 'Walker_Nav_Primary' not found in D:\wamp\www\SgsOnline\wp-content\themes\storefront\inc\structure\header.php on line 65

This is oblivious wrong because it is looking in the wrong place for the class.

If anyone has any experience with this would be great

Regards

via Chebli Mohamed