decoder/wavpack: fix WVC file support

The wrong variable was used.  This has been broken by commit
4eeea640f4 seven years ago - and nobody as noticed, d'oh!

src/decoder/plugins/WavpackDecoderPlugin.cxx

@@ -521,7 +521,7 @@ wavpack_open_wvc(DecoderClient &client, const char *uri)
 	};
 
 	try {
-		return client.OpenUri(uri);
+		return client.OpenUri(wvc_url);
 	} catch (...) {
 		return nullptr;
 	}