Better method is to add only css code:
.custom-logo-link img {
max-width: 100%;
height: auto;
}
But if you need to add a custom class to logo you can add a filter:
// Add custom class to logo.
add_filter( 'get_custom_logo', 'change_logo_class' );
function change_logo_class( $html ) {
$html = str_replace( 'custom-logo-link', 'header-brand', $html );
$html = str_replace( 'custom-logo', 'img-responsive', $html );
return $html;
}