Fix builtin class reference

[h3xx]

Fix fatal:

PHP Fatal error:  Uncaught Error: Class "Drush\Preflight\RuntimeException" not found in /Users/dchurch/dotfiles/drupal/drush/src/Preflight/Preflight.php:217
Stack trace:
#0 /Users/dchurch/dotfiles/drupal/drush/src/Runtime/Runtime.php(84): Drush\Preflight\Preflight->loadSymfonyCompatabilityAutoloader()
#1 /Users/dchurch/dotfiles/drupal/drush/src/Runtime/Runtime.php(51): Drush\Runtime\Runtime->doRun(Array, Object(Symfony\Component\Console\Output\ConsoleOutput))
#2 /Users/dchurch/dotfiles/drupal/drush/drush.php(72): Drush\Runtime\Runtime->run(Array)
#3 {main}
  thrown in /Users/dchurch/dotfiles/drupal/drush/src/Preflight/Preflight.php on line 217