bug symfony#16902 [2.8][Security] Fix a Polyfill import statement in StringUtils (magnetik)

This PR was merged into the 2.8 branch.

Discussion
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[2.8][Security] Fix a Polyfill import statement in StringUtils

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

Commits
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2a9fa81 [Security] Fix a Polyfill import statement in StringUtils

Author: nicolas-grekas

src/Symfony/Component/Security/Core/Util/StringUtils.php

@@ -13,7 +13,7 @@
 
 @trigger_error('The '.__NAMESPACE__.'\\StringUtils class is deprecated since version 2.8 and will be removed in 3.0. Use hash_equals() instead.', E_USER_DEPRECATED);
 
-use Symfony\Component\Polyfill\Util\Binary;
+use Symfony\Polyfill\Util\Binary;
 
 /**
  * String utility functions.