hardcoded path to ext library prevents use in multi-site installations

While attempting to install Take Control, I noticed that my sites/{sitename}/libraries/extjs/3.4.0 directory wasn't being read.

The notice error at the top of my screen appeared to show that Take Control was looking in sites/all/libraries - I'm a multi-site user (in this case), and want my ext library installed into my site directory, not sites/all.

I'll attach a short patch that should fix this, by amending includes/filesystem.inc to use conf_path() to get the current site directory.

Thanks!