Koma Export: rename org-export-latex to org-latex for package lists

* ox-koma-letter.el (org-koma-letter-template): rename
org-export-latex to org-latex for package lists.

These variable names were changed with the merge of the new exporter.

contrib/lisp/ox-koma-letter.el

@@ -193,8 +193,8 @@ holding export options."
            (org-e-latex--guess-inputenc
             (org-splice-latex-header
              document-class-string
-             org-export-latex-default-packages-alist ; defined in org.el
-             org-export-latex-packages-alist nil ; defined in org.el
+             org-latex-default-packages-alist ; defined in org.el
+             org-latex-packages-alist nil ; defined in org.el
              (plist-get info :latex-header-extra)))
            info)))))
    ;; Define "From" data.