org-macro: Require vc in vc-modified-time

* lisp/org-macro.el (org-macro--vc-modified-time): Not all the vc
components needed are autoloaded, allowing for the error:
"vc-call-backend: Symbol’s value as variable is void: vc-log-view-type".
To prevent this issue from cropping up, vc is now required at the start
of `org-macro--vc-modified-time'.

lisp/org-macro.el

@@ -370,6 +370,7 @@ Return value as a string."
       value)))
 
 (defun org-macro--vc-modified-time (file)
+  (require 'vc) ; Not everything we need is autoloaded.
   (save-window-excursion
     (when (vc-backend file)
       (let ((buf (get-buffer-create " *org-vc*"))