Added missing use statemenet

We should use Symfony\Component\Translation\Exception\RuntimeException

Author: Nyholm

src/Symfony/Component/Translation/Dumper/FileDumper.php

@@ -13,6 +13,7 @@
 
 use Symfony\Component\Translation\MessageCatalogue;
 use Symfony\Component\Translation\Exception\InvalidArgumentException;
+use Symfony\Component\Translation\Exception\RuntimeException;
 
 /**
  * FileDumper is an implementation of DumperInterface that dump a message catalogue to file(s).