fix compiler warning in org-exp.el

* lisp/org-exp.el (org-export-latex-listings-w-names): fix compiler
  warning in org-exp.el

lisp/org-exp.el

@@ -2146,6 +2146,7 @@ in the list) and remove property and value from the list in LISTVAR."
 (defvar org-export-latex-verbatim-wrap) ;; defined in org-latex.el
 (defvar org-export-latex-listings) ;; defined in org-latex.el
 (defvar org-export-latex-listings-langs) ;; defined in org-latex.el
+(defvar org-export-latex-listings-w-names) ;; defined in org-latex.el
 
 (defun org-export-format-source-code-or-example
   (backend lang code &optional opts indent caption)