QL-DIST:CLEAN deletes too much on LispWorks

[quicklisp]

Reported by John DeSoi on the quicklisp list.

https://groups.google.com/forum/#!topic/quicklisp/Gq4InyrVwj8

Everything in software/ is deleted, rather than the unneeded files.

[quicklisp]

Fix released.