org-clock: Autoload `org-clock-modify-effort-estimate'

* lisp/org-clock.el (org-clock-modify-effort-estimate): Autoload the
command.  It is used in the agenda menu without loading org-clock, so
we must autoload it.  Also, some users want to use the command before
clockin-in.

Link: https://orgmode.org/list/2fb82950-316e-4ebb-bb8b-a65b45e671eb@Spark

lisp/org-clock.el

@@ -779,6 +779,7 @@ previous clocking intervals."
 		60)))
     (+ currently-clocked-time (or org-clock-total-time 0))))
 
+;;;###autoload
 (defun org-clock-modify-effort-estimate (&optional value)
   "Add to or set the effort estimate of the item currently being clocked.
 VALUE can be a number of minutes, or a string with format hh:mm or mm.