bug#5156: M-x font-show-log fails

[Kenichi Handa]

In article <7rhbs1tht1.fsf@xxxxxxxxxxxxxxxxx>, Glenn Morris <rgm@xxxxxxx> 
writes:

> EMACS_FONT_LOG=1 ./src/emacs -q --no-site-file
> M-x font-show-log

> Fails with "Unknown button type `help-xref'".

> Debugger entered--Lisp error: (error "Unknown button type `help-xref'")

I've just fixed it by require help-mode instead of help-fns
in mule-diag.el.

---
Kenichi Handa
handa@xxxxxxxx