Add support for waiting for FCP driver initialisation

When a card using the FCP driver is added at runtime, we need to wait
for fcp-server to finish creating all the controls before we attempt
to enumerate them.

src/iface-waiting.h

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: 2025 Geoffrey D. Bennett <g@b4.vu>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include "alsa.h"
+
+GtkWidget *create_iface_waiting_main(struct alsa_card *card);