Check $wgEnableWikiaHomePageExt when trying to use WikiaHomePageHelper class

This code should only be run when WikiaHomePage extension is included.

Fixes the following fatal:

```
Jan 23 10:01:32 ap-s25 apache2[21096]: PHP Fatal error:  Class 'WikiaHomePageHelper' not found (URL: pt-br.crystalpedia.wikia.com/wikia.php?controller=WikiaSearch&method=getPages&ids=630&format=json) in /usr/wikia/slot1/code/extensions/wikia/Search/classes/IndexService/WikiPromoData.php on line 29
```