Inform compiler about a variable that will exist at run time

* lisp/org-timer.el (org-clock-sound): Silence compiler.

lisp/org-timer.el

@@ -370,6 +370,8 @@ VALUE can be `on', `off', or `pause'."
       (message "%d minute(s) %d seconds left before next time out"
 	       rmins rsecs))))
 
+(defvar org-clock-sound)
+
 ;;;###autoload
 (defun org-timer-set-timer (&optional opt)
   "Prompt for a duration and set a timer.