<?php if (!is_singular()) { ?>
<link rel="prev" href="<?php echo get_previous_posts_page_link(); ?>" />
<link rel="next" href="<?php echo get_next_posts_page_link(); ?>" />
<?php } else {?>
<link rel="prev" href="<?php echo get_permalink(get_adjacent_post($in_same_cat = false, $excluded_categories = '', $previous = true)->ID); ?>" />
<link rel="next" href="<?php echo get_permalink(get_adjacent_post($in_same_cat = false, $excluded_categories = '', $previous = false)->ID); ?>" />
<?php } ?>