Update upload/admin/view/template/catalog/filter_form.tpl

AddFilter() is a built-in Javascript function in IE9, so IE users can not add rows to the filter table.

Renaming it to something else fixes the problem.

http://forum.opencart.com/viewtopic.php?f=161&t=93332&start=100#p383737
Merge pull request #495 from jcsmithy/patch-2

Update upload/admin/view/template/catalog/filter_form.tpl