org-macro: Adjust for compatibility with latest vc (bug#51365)

* lisp/org-macro.el (org-macro--vc-modified-time): Wrap file in a list
when passing to vc-call to avoid a type error starting with Emacs 28.

Before Emacs's 3572613550f (Fix vc-git-state for filenames with
wildcards, 2021-08-15), a string could be given to vc-git-print-log as
the FILES argument because the argument was passed directly as
vc-git-command's FILE-OR-LIST, which as the name suggests, accepts
either a string or a list.  After 3572613550f, passing a string leads
to a type error.

TINYCHANGE

lisp/org-macro.el

@@ -368,7 +368,7 @@ Return value as a string."
 	    date)
 	(unwind-protect
 	    (progn
-	      (vc-call print-log file buf nil nil 1)
+	      (vc-call print-log (list file) buf nil nil 1)
 	      (with-current-buffer buf
 		(vc-exec-after
 		 (lambda ()