Fix an error: void-variable dv in Emacs-30.0.50

[dvzubarev]

In recent Emacs-30, modeline and headerline has stopped working. There were many errors in the Messages:

Error during redisplay: (eval (dirvish-path-ml dv) t) signaled (void-variable dv) 
Error during redisplay: (eval (dirvish-sort-ml dv) t) signaled (void-variable dv)

[jimeh]

For whatever it's worth, this change resolves the issue for me too.

[jimeh]

@alexluigit Any chance this could get merged soon? :)

[Thaodan]

Could you add "Resolves #260" to the commit message?