<?php
  $single = (is_single() ? TRUE : FALSE);
  if ($single){
    $prevPost = get_previous_post();
    if (!empty( $prevPost )){ ?>
      <link rel="prev" href="<?php echo get_permalink($prevPost->ID); ?>"/>
    <?php }
    $nextPost = get_next_post();
    if (!empty( $nextPost )) { ?>
      <link rel="next" href="<?php echo get_permalink($nextPost->ID); ?>"/>
    <?php }
  }
?>