Fix error running timer `wl-biff-event-handler'

Cf. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=768910
> Dear Maintainer,
>      I keep seeing timer errors from wl-biff:
>      
>      Error running timer `wl-biff-event-handler':
>         (wrong-type-argument number-or-marker-p t)
> 
> This is because t is passed to timer-set-idle-time instead of a number
> of seconds.
> 
> Here is a patch to fix it: