Allow all users who can alter performance settings access to clear js cache function. (Patch attached)

The attached patch allows users other than admin (user id = 1) the ability to flush the cache. I almost never log in to my site as user id 1, so I can't flush the cache unless I switch users. This mod allows any user having permission that allows them to access the performance settings page to clear the cache using the link provided on that page.

Basically, any user with 'administer site configuration' perms can see the performance settings, and therefore, should be allowed to clear the cache.