src: add ofono-iface

Start work on new ofono interface. So far, this detects ofono on dbus
and complains if both mm and ofono are running.

src/ofono-iface.h

@@ -0,0 +1,12 @@
+/*
+ * Copyright (C) 2020 Oliver Smith <ollieparanoid@postmarketos.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "manager.h"
+
+void ofono_iface_init(struct EG25Manager *data);
+void ofono_iface_destroy(struct EG25Manager *data);