logwatcher: Use cond_signal instead of cond_broadcast on watcher_cond

as there is only one thread waiting

src/fellow_log.c

@@ -5391,7 +5391,7 @@ fellow_logs_rewrite(struct fellow_fd *ffd,
 	AZ(pthread_mutex_lock(&ffd->logmtx));
 	ffd->rewriting = 0;
 	if (ffd->watcher_running)
-		AZ(pthread_cond_broadcast(&ffd->watcher_cond));
+		AZ(pthread_cond_signal(&ffd->watcher_cond));
 	AZ(pthread_mutex_unlock(&ffd->logmtx));
 	ffd->diag("... done: %fs\n", VTIM_mono() - t0);
 }
@@ -5761,7 +5761,7 @@ static void
 fellow_logwatcher_kick_locked(struct fellow_fd *ffd)
 {
 	if (ffd->phase == FP_OPEN && ffd->watcher_running)
-		AZ(pthread_cond_broadcast(&ffd->watcher_cond));
+		AZ(pthread_cond_signal(&ffd->watcher_cond));
 }
 
 // not unter logmtx
@@ -5794,7 +5794,7 @@ fellow_logwatcher_fini(struct fellow_fd *ffd)
 
 	AZ(pthread_mutex_lock(&ffd->logmtx));
 	ffd->watcher_running = 0;
-	AZ(pthread_cond_broadcast(&ffd->watcher_cond));
+	AZ(pthread_cond_signal(&ffd->watcher_cond));
 	AZ(pthread_mutex_unlock(&ffd->logmtx));
 	AZ(pthread_join(ffd->watcher_thread, NULL));
 	AZ(pthread_cond_destroy(&ffd->watcher_cond));