Cannot modify header information – headers already sent

To add the sidebar in my twentyeleven theme I’ve added functions.php + single.php files to my twentyeleven-child theme. But now it gives the following error:

Warning: Cannot modify header information – headers already sent by (output started at /home2/voiceact/public_html/oop/wp-content/themes/twentyeleven-child/functions.php:2) in /home2/voiceact/public_html/oop/wp-includes/pluggable.php on line 881

My functions.php in the child theme:

<?php
function remove_twentyeleven_body_classes() {
    remove_filter( 'body_class', 'twentyeleven_body_classes' );
}
add_action( 'after_setup_theme', 'remove_twentyeleven_body_classes' );
?>

My single.php in the child theme:

<?php
/**
 * The Template for displaying all single posts.
 *
 * @package WordPress
 * @subpackage Twenty_Eleven
 * @since Twenty Eleven 1.0
 */

get_header(); ?>

		<div id="primary">
			<div id="content" role="main">

				<?php while ( have_posts() ) : the_post(); ?>

					<nav id="nav-single">
						<h3 class="assistive-text"><?php _e( 'Post navigation', 'twentyeleven' ); ?></h3>
						<span class="nav-previous"><?php previous_post_link( '%link', __( '<span class="meta-nav">&larr;</span> Previous', 'twentyeleven' ) ); ?></span>
						<span class="nav-next"><?php next_post_link( '%link', __( 'Next <span class="meta-nav">&rarr;</span>', 'twentyeleven' ) ); ?></span>
					</nav><!-- #nav-single -->

					<?php get_template_part( 'content', 'single' ); ?>

					<?php comments_template( '', true ); ?>

				<?php endwhile; // end of the loop. ?>

			</div><!-- #content -->
		</div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>