BUILD: wdt: only test for SI_TKILL when compiled with thread support

SI_TKILL is not necessarily defined on older systems and is used only
with the pthread_kill() call a few lines below, so it should also be
subject to the USE_THREAD condition.

src/wdt.c

@@ -101,12 +101,12 @@ void wdt_handler(int sig, siginfo_t *si, void *arg)
 
 		/* No doubt now, there's no hop to recover, die loudly! */
 		break;
-
+#ifdef USE_THREAD
 	case SI_TKILL:
 		/* we got a pthread_kill, stop on it */
 		thr = tid;
 		break;
-
+#endif
 	default:
 		/* unhandled other conditions */
 		return;