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

[Glenn Morris]

In the current CVS trunk:

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'")
  signal(error ("Unknown button type `help-xref'"))
  error("Unknown button type `%s'" help-xref)
  define-button-type(list-charset-chars :supertype help-xref help-function 
list-charset-chars help-echo "mouse-2, RET: show table of characters for this 
character set")
  
byte-code("\300\301\302\303\304!\305\306\307\310&\210\300\311\312\313\314\311\302\315&\207"
 [define-button-type sort-listed-character-sets help-echo purecopy "mouse-2, 
RET: sort on this column" face bold action #[(button) "\301\302\303\"!\207" 
[button sort-listed-character-sets button-get sort-key] 4] list-charset-chars 
:supertype help-xref help-function "mouse-2, RET: show table of characters for 
this character set"] 8)
  execute-extended-command(nil)
  call-interactively(execute-extended-command nil nil)