Implement `org-decode-time' for backward compatibility

* lisp/org-compat.el (org-decode-time): New function.
* lisp/org-clock.el (org-clock-get-sum-start): Use new function.

Emacs 24's `decode-time' doesn't accept two arguments.

lisp/org-clock.el

@@ -1440,7 +1440,7 @@ The time is always returned as UTC."
       (current-time))
      ((equal cmt "today")
       (setq org--msg-extra "showing today's task time.")
-      (let* ((dt (decode-time nil t))
+      (let* ((dt (org-decode-time nil t))
 	     (hour (nth 2 dt))
 	     (day (nth 3 dt)))
 	(if (< hour org-extend-today-until) (setf (nth 3 dt) (1- day)))

lisp/org-compat.el

@@ -61,6 +61,12 @@
   (defalias 'format-message 'format)
   (defalias 'gui-get-selection 'x-get-selection))
 
+(defun org-decode-time (&optional time zone)
+  "Backward-compatible function for `decode-time'."
+  (if (< emacs-major-version 25)
+      (decode-time time)
+    (decode-time time zone)))
+
 (unless (fboundp 'directory-name-p)
   (defun directory-name-p (name)
     "Return non-nil if NAME ends with a directory separator character."