desktop-file-utils: don't build /usr/share/emacs/site-lisp/desktop-entry-mode.el

* fixes:
desktop-file-utils-0.18: desktop-file-utils: Files/directories were installed but not shipped
  /usr/share/emacs
  /usr/share/emacs/site-lisp
  /usr/share/emacs/site-lisp/desktop-entry-mode.el
  /usr/share/emacs/site-lisp/desktop-entry-mode.elc
  when emacs is available

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>