org-pcomplete.el: Fix bug in `pcomplete/org-mode/file-option'.

* org-pcomplete.el (pcomplete/org-mode/file-option): Fix bug
in `pcomplete/org-mode/file-option'.

Before this fix, completing #+ATTR worked only once, as
`org-additional-option-like-keywords' was modified.

lisp/org-pcomplete.el

@@ -144,7 +144,7 @@ When completing for #+STARTUP, for example, this function returns
 			      (if (string-match "^#\\+\\([A-Z_]+:?\\)" x)
 				  (match-string 1 x)))
 			    (org-split-string (org-get-current-options) "\n"))
-		    org-additional-option-like-keywords)))))
+		    (copy-sequence org-additional-option-like-keywords))))))
    (substring pcomplete-stub 2)))
 
 (defvar org-startup-options)