Use function argument instead of dynamically scoped symbol

* org-latex.el (org-export-latex-preprocess): Use function argument
instead of dynamically scoped symbol.

Export options are passed to org-export-preprocess-string which passes
them down to backend specific preprocessor. The :footnotes option is
included.

lisp/org-latex.el

@@ -2430,7 +2430,7 @@ The conversion is made depending of STRING-BEFORE and STRING-AFTER."
      (replace-match "")))
 
   ;; When converting to LaTeX, replace footnotes.
-  (when (plist-get opt-plist :footnotes)
+  (when (plist-get parameters :footnotes)
     (goto-char (point-min))
     (let (ref)
       (while (setq ref (org-footnote-get-next-reference))