bug#49677: Compiler warnings for bug-reference.el

[Arash Esbati]

Hi all,

when native compiling bug-reference.el, the compiler says:

    Compiling 
path-to/emacs/share/emacs/28.0.50/lisp/progmodes/bug-reference.el...

    In bug-reference--try-setup-gnus-article:
    bug-reference.el:350:14: Warning: reference to free variable 
`bug-reference-mode'

    In bug-reference-try-setup-from-rmail:
    bug-reference.el:396:14: Warning: reference to free variable 
`bug-reference-mode'

    In bug-reference--run-auto-setup:
    bug-reference.el:506:13: Warning: reference to free variable 
`bug-reference-mode'

The variable is defined through the minor-mode at the end of the file.
I suggest to add the following

(defvar bug-reference-mode)

in line 100 just before

(defvar bug-reference-prog-mode)

which should fix the issue.

I'm running Emacs 28 repo version 13b247c3c4.

Best, Arash