bug symfony#21126 Added missing use statemenet (Nyholm)

This PR was merged into the 3.3-dev branch.

Discussion
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Added missing use statemenet

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

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

Commits
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f9848be Added missing use statemenet

Author: nicolas-grekas

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).