filter/Volume: add volume_filter_prepare()

Circumvent filter_new() with a fake ConfigBlock.

src/filter/plugins/VolumeFilterPlugin.cxx

@@ -78,6 +78,12 @@ const FilterPlugin volume_filter_plugin = {
 	volume_filter_init,
 };
 
+PreparedFilter *
+volume_filter_prepare() noexcept
+{
+	return new PreparedVolumeFilter();
+}
+
 unsigned
 volume_filter_get(const Filter *_filter) noexcept
 {

src/filter/plugins/VolumeFilterPlugin.hxx

@@ -20,8 +20,12 @@
 #ifndef MPD_VOLUME_FILTER_PLUGIN_HXX
 #define MPD_VOLUME_FILTER_PLUGIN_HXX
 
+class PreparedFilter;
 class Filter;
 
+PreparedFilter *
+volume_filter_prepare() noexcept;
+
 unsigned
 volume_filter_get(const Filter *filter) noexcept;