filter the available post types in the internal linking

rkv-filter-link-types.php

<?php
/**
 * filter the query args in `wp_link_query`
 * to only show specific post types
 * 
 * @param  array      $query     the default args
 * @return array      $query     the args with our new post type array
 */
function rkv_filter_link_types( $query ) {

	// set an array of the types I want to include
	$query['post_type']	= array( 'post', 'page' );

	// return query
	return $query;

}

add_filter( 'wp_link_query_args', 'rkv_filter_link_types' );