How do I include SVG file used as featured image?

First, you don’t need the URL but the server path to the file, you can get it with get_attached_file function, passing any WordPress attachment ID as a parameter. Then you need to load the contents of that SVG file directly, via file_get_contents function and echo it out to the page.

$thumbnail_id   = get_post_thumbnail_id( get_the_ID() );
$thumbnail_path = get_attached_file( $thumbnail_id );
echo file_get_contents( $thumbnail_path );