Add the jquery script as a dependency for your jquery-ticker.js and site.js files within the wp_enqueue_script function.
Example:
function bbc_frontend_script() {
wp_enqueue_script('ticker', plugins_url("/js/jquery-ticker.js", __FILE__), array( 'jquery' ) );
wp_enqueue_script('sitejs', plugins_url("/js/site.js", __FILE__), array( 'jquery', 'ticker' ) );
}
add_action( 'wp_enqueue_scripts', 'bbc_frontend_script' );
You also don’t have to enqueue jquery separately if you define it as a dependency.
Edited:
Added missing ) at end of enqueue statements.