Where to put fix for #22317, mh-e: wrong usage of cl-flet

[Bill Wohler]

I've applied Katsumi's fix to MH-E and have been using it for a while
now. I haven't noticed any difference.

Please consider the following short patch and recommend whether I should
1) apply it to emacs-25, 2) apply it to master, or 3) apply it to master
with a note to apply it to emacs-25 after the release. If 3), please
suggest specific text that catches your attention.

[Eli Zaretskii]

Date: Sun, 01 May 2016 19:19:48 -0700
I've applied Katsumi's fix to MH-E and have been using it for a while
now. I haven't noticed any difference.
Please consider the following short patch and recommend whether I should
1) apply it to emacs-25, 2) apply it to master, or 3) apply it to master
with a note to apply it to emacs-25 after the release. If 3), please
suggest specific text that catches your attention.

Looks safe for emacs-25, thanks.