functions.php

<?php

/**
 * Exclude Listings from Link Search
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/exclude-listings-from-link-search
 *
 * @param array $query, wp_query arguments
 * @return array $query
 */
function be_exclude_listings_from_link_search( $query ) {
	$query['post_type'] = array_diff( $query['post_type'], array( 'listing' ) );
	return $query;
}
add_filter( 'wp_link_query_args', 'be_exclude_listings_from_link_search' );