ox-koma-letter: Support #+LATEX_COMPILER

* ox-koma-letter.el (org-koma-letter-template): Insert compiler.

contrib/lisp/ox-koma-letter.el

@@ -616,6 +616,8 @@ holding export options."
    ;; Time-stamp.
    (and (plist-get info :time-stamp-file)
         (format-time-string "%% Created %Y-%m-%d %a %H:%M\n"))
+   ;; LaTeX compiler
+   (org-latex--insert-compiler info)
    ;; Document class and packages.
    (org-latex--make-preamble info)
    ;; Settings.  They can come from three locations, in increasing