How to load file only in single post page through function.php?

is_single function checks for $wp_query global variable is loading single post of any post type or not , $wp_query is set after running query_posts function that loaded after wp action hook.

So you should call this function in the right order ( hook )

add_action( 'wp', 'include_single_lazy_load' );

function include_single_lazy_load() {
    if( is_single() ) {
        require_once(dirname(__FILE__) . '/inc/lazy-load.php');