Silence byte-compiler on Emacs 28/29

[skangas]

This fixes a byte-compiler warning in Emacs 28:

Compiling file /home/skangas/wip/emacs-packages/emacs-winum/winum.el at Sat Jan  1 21:22:53 2022
Entering directory ‘/home/skangas/wip/emacs-packages/emacs-winum/’
winum.el:239:3: Warning: Use keywords rather than deprecated positional
    arguments to `define-minor-mode'

Thanks!

[skangas]

@deb0ch friendly ping

[pataquets]

@deb0ch Anything else needed in this PR to get merged?

[skangas]

@deb0ch Friendly ping.

Are you still maintaining this package? I'm happy to take over as maintainer if you don't have time or have lost interest.