Tax query with multiple terms in pre_get_posts

Just use:

'terms' => $rt_cat_id

I’ll work for both array and single-based values.

Or you can simplify your code as follows:

if( isset( $_GET[ 'listing_cat' ] ) ) {
   $tax_query[] = array(
      'taxonomy' => 'listing_category',
      'field' => 'id',
      'terms' => $_GET[ 'listing_cat' ]
   );
}