conf: const pointers in block get functions

All config_get_block_*() functions should accept constant config_param
pointers.

src/output/null_plugin.c

@@ -32,7 +32,7 @@ struct null_data {
 static void *
 null_init(G_GNUC_UNUSED struct audio_output *audio_output,
 	  G_GNUC_UNUSED const struct audio_format *audio_format,
-	  G_GNUC_UNUSED struct config_param *param)
+	  G_GNUC_UNUSED const struct config_param *param)
 {
 	struct null_data *nd = g_new(struct null_data, 1);