client: fix compile warning on platforms where pfds->fd is not an int

It's apparently a long on some platforms.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/client.c b/client.c
index 155eb72..b20d6dc 100644
--- a/client.c
+++ b/client.c
@@ -1462,7 +1462,7 @@
 
 			client = find_client_by_fd(pfds[i].fd);
 			if (!client) {
-				log_err("fio: unknown client fd %d\n", pfds[i].fd);
+				log_err("fio: unknown client fd %ld\n", (long) pfds[i].fd);
 				continue;
 			}
 			if (!fio_handle_client(client)) {