Fixed Purge cache button not working because of a missing form

Revised version by infograf768. Closes #3490.

JC Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33000&start=0
Duplicate reports:
http://issues.joomla.org/tracker/joomla-cms/3284
http://issues.joomla.org/tracker/joomla-cms/2738