decoder_plugin: pass struct config_param to init() method

Preparing for per-plugin configuration sections in mpd.conf.

src/decoder_list.c

@@ -190,7 +190,7 @@ void decoder_plugin_init_all(void)
 	for (unsigned i = 0; i < num_decoder_plugins; ++i) {
 		const struct decoder_plugin *plugin = decoder_plugins[i];
 
-		if (decoder_plugin_init(plugin))
+		if (decoder_plugin_init(plugin, NULL))
 			decoder_plugins_enabled[i] = true;
 	}
 }