Fix EZP-21420: Added zetacomponents/event-log to composer.json

When the Zeta Components are used (installed through composer, not available in lib/ezc or through PEAR), running the PHPUnit tests with an activated extension (e.g. ezcomments) fails with the message 

```
Fatal error: Class 'ezcLog' not found in /../vendor/zetacomponents/debug/src/debug.php on line 145
```

Adding `zetacomponents/event-log` to the composer.json file fixes this.

https://jira.ez.no/browse/EZP-21408