[Bug] org-persist.el: xdg.el is unavailable in emacs-25.2

[Max Nikulin]

Org-mode compilation fails with the following errors if emacs-25.2.2 is 
used (Ubuntu-18.04 LTS bionic)

> Compiling /home/ubuntu/org-mode/lisp/ox-ascii.el...
>
> In toplevel form:
> ox-ascii.el:31:1:Error: Cannot open load file: No such file or directory, xdg
> Compiling /home/ubuntu/org-mode/lisp/ox-beamer.el...
>
> In toplevel form:
> ox-beamer.el:34:1:Error: Cannot open load file: No such file or directory, xdg
>
> ...

org.el in the main branch still declares

> ;; Package-Requires: ((emacs "25.1"))

but it seems xdg.el used by org-persist.el was added in emacs-26.0.

I do not actually use emacs-25.2, but it happened that I have a test 
container for Org based on Ubuntu-18.04.

Either org-persist and org-compat should be updated with some 
compatibility code or required emacs version should be updated once more.