decoder_plugin: pass struct config_param to init() method

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

src/decoder/wildmidi_plugin.c

@@ -30,7 +30,7 @@ enum {
 };
 
 static bool
-wildmidi_init(void)
+wildmidi_init(G_GNUC_UNUSED const struct config_param *param)
 {
 	int ret;