org-table: Turn `org-table-current-column' into a command again

* lisp/org-table.el (org-table-current-column): Add `interactive'
  spec.

This function is called as a command by "Which column?" menu entry.

Reported-by: Chunyang Xu <mail@xuchunyang.me>
<http://permalink.gmane.org/gmane.emacs.orgmode/111785>

lisp/org-table.el

@@ -1317,6 +1317,7 @@ is always the old value."
 
 (defun org-table-current-column ()
   "Find out which column we are in."
+  (interactive)
   (save-excursion
     (let ((column 0) (pos (point)))
       (beginning-of-line)