[HttpKernel] Fix gitignore

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

The kernel rootDir is Fixtures:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/Tests/KernelTest.php#L787-L789

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/Tests/KernelTest.php#L800-L802