GAsyncQueue: Add copy-func and free-func annotations

GAsyncQueue is an opaque struct which supports some memory management through g_async_queue_ref and g_async_queue_unref. As those functions are currently not annotated language bindings have to use heuristics or implement the memory management manually. This MR uses the copy-func and free-func annotations to give language bindings some hints how to copy / free an instance of this struct.