fix CLISP function for directory deletion

symbol was incorrect
*** - READ from #<INPUT BUFFERED FILE-STREAM CHARACTER #P"/home/c4b3z0n/quicklisp/dists/quicklisp/software/cl-fad-20180430-git/fad.lisp" @308>: #<PACKAGE EXT> has no external symbol with name "DELETE-DIR"
should be ext:delete-directory

fad.lisp

@@ -305,7 +305,7 @@ might be removed instead!  This is currently fixed for SBCL and CCL."
                                     (unless ok
                                       (error "~@<Error deleting ~S: ~A~@:>"
                                              file (unix:get-unix-error-msg errno))))
-                           #+:clisp (ext:delete-dir file)
+                           #+:clisp (ext:delete-directory file)
                            #+:openmcl (cl-fad-ccl:delete-directory file)
                            #+:cormanlisp (win32:delete-directory file)
                            #+:ecl (si:rmdir file)