server: fix warnings on some platforms on incompatible pointer type

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/server.c b/server.c
index b6961fd..dc70616 100644
--- a/server.c
+++ b/server.c
@@ -1138,7 +1138,7 @@
 	struct fio_net_cmd cmd;
 	struct iovec iov[2];
 
-	iov[0].iov_base = &cmd;
+	iov[0].iov_base = (void *) &cmd;
 	iov[0].iov_len = sizeof(cmd);
 	iov[1].iov_base = (void *) buf;
 	iov[1].iov_len = size;