Added ext-iconv as dependency to composer.json

I got this error when iconv wasn't loaded:

PHP Fatal error:  Call to undefined function Ddd\Slug\Infra\Transliterator\iconv() in /home/r1pp3rj4ck/.composer/vendor/ddd/slug/Ddd/Slug/Infra/Transliterator/LatinTransliterator.php on line 38

Enabling iconv extension fixed the issue, so I put it here as a dependency.