id: Fix behavior when `org-id-extra-files' is a symbol

* lisp/org-id.el (org-id-update-id-locations): When
`org-id-extra-files' is a symbol, it should be evaluated as a
variable.

In 9865e6bd8be65229be4eac4f459f62e47fab2be737a5020bb,
`org-id-update-id-locations' was rewritten, and the functionality to
allow the variable `org-id-extra-files' to be a symbol that references
another variable was removed.  This change restores that
functionality.

lisp/org-id.el

@@ -495,7 +495,9 @@ When FILES is given, scan also these files."
                     ;; Agenda files and all associated archives.
                     (org-agenda-files t org-id-search-archives)
                     ;; Explicit extra files.
-                    (unless (symbolp org-id-extra-files) org-id-extra-files)
+                    (if (symbolp org-id-extra-files)
+			(symbol-value org-id-extra-files)
+		      org-id-extra-files)
                     ;; All files known to have IDs.
                     org-id-files
                     ;; Additional files from function call.