Fix for issue 11049

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #11049
| License       | MIT
| Doc PR        | 

See the original ticket.
I've added support for the new `Symfony\Component\Validator\Context\ExecutionContextInterface` in `Symfony\Component\Validator\ConstraintValidator`, so the new Validation api can be used.
hmm, changing this interface is indeed a BC break.

@webmozart what would be the right way to handle the 2.5 API there ?