[2.6] [Security] Fixed ExpressionVoter - addExpressionLanguageProvider

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | ---
| License       | MIT
| Doc PR        | ---


Fix missing addExpressionLanguageProvider in [ExpressionVoter](https://github.com/symfony/symfony/blob/2.6/src/Symfony/Component/Security/Core/Authorization/Voter/ExpressionVoter.php), used by [AddExpressionLanguageProvidersPass](https://github.com/symfony/symfony/blob/2.6/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Compiler/AddExpressionLanguageProvidersPass.php) to add expression providers.