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.

src/wdt.c

@@ -60,7 +60,7 @@ void wdt_handler(int sig, siginfo_t *si, void *arg)
 		 * the thread number from there. Note: this thread might
 		 * continue to execute in parallel.
 		 */
-		thr = si->si_int;
+		thr = si->si_value.sival_int;
 
 		/* cannot happen unless an unknown timer tries to play with our
 		 * nerves. Let's die for now if this happens.