Add 'rel="next"' and 'rel="prev"' attribute to the next and previous links in the paginate_links function in WordPress.

functions.php

<?php
/**
 * Alter the pagination next and prev links to have rel.
 *
 * @param string $r    HTML output.
 * @param array  $args An array of arguments. See paginate_links() for information on accepted arguments.
 *
 * @return sting - The HTML output of the pagination.
 */
function scaffolding_add_rel_pagination( $r, $args ) {

	$r = str_replace( 'class="prev', 'rel="prev" class="prev', $r );
	$r = str_replace( 'class="next', 'rel="next" class="next', $r );

	return $r;
}
add_filter( 'paginate_links_output', 'scaffolding_add_rel_pagination', 20, 2 );