Update plugins/fabrik_element/databasejoin/databasejoin.php

I have fixed a bug that makes database-join caused by a a change in the parent's variable name that was not reflected in this class. Check the link below for more details.

http://fabrikar.com/forums/showthread.php?t=31983
Merge pull request #652 from TwistedHardware/joomla3

Update plugins/fabrik_element/databasejoin/databasejoin.php