* .dir-locals.el: Fix whitespace-mode configuration.

.dir-locals.el

-((nil . ((whitespace-action . '(auto-cleanup))
-	 (whitespace-style . '(trailing lines-tail))
-	 (require-final-newline . t)))
+((nil . ((whitespace-action auto-cleanup)
+	 (whitespace-style face empty trailing lines-tail)
+	 (require-final-newline . t)
+	 (mode . whitespace))))
  (c++-mode . ((c-default-style . "gnu")
 	      (indent-tab-mode . t)
 	      (tab-width . 8)