BUILD: watchdog: use si_value.sival_int, not si_int for the timer's value

BUILD: watchdog: use si_value.sival_int, not si_int for the timer's value

Bah, the linux manpage suggests to use si_int but it's a fake, it's only
a define on sigval.sival_int where sigval is defined as si_value. Let's
use si_value.sival_int, at least it builds on both Linux and FreeBSD. It's
likely that this code will have to be limited to a small subset of OSes
if it causes difficulties like this.