db/update/Container: move SupportsContainerSuffix() to struct DecoderPlugin

src/db/update/Container.cxx

@@ -55,21 +55,13 @@ UpdateWalk::MakeDirectoryIfModified(Directory &parent, const char *name,
 	return directory;
 }
 
-static bool
-SupportsContainerSuffix(const DecoderPlugin &plugin,
-			const char *suffix) noexcept
-{
-	return plugin.container_scan != nullptr &&
-		plugin.SupportsSuffix(suffix);
-}
-
 bool
 UpdateWalk::UpdateContainerFile(Directory &directory,
 				const char *name, const char *suffix,
 				const StorageFileInfo &info) noexcept
 {
 	const DecoderPlugin *_plugin = decoder_plugins_find([suffix](const DecoderPlugin &plugin){
-			return SupportsContainerSuffix(plugin, suffix);
+			return plugin.SupportsContainerSuffix(suffix);
 		});
 	if (_plugin == nullptr)
 		return false;

src/decoder/DecoderPlugin.hxx

@@ -225,6 +225,10 @@ struct DecoderPlugin {
 	 */
 	gcc_pure gcc_nonnull_all
 	bool SupportsMimeType(const char *mime_type) const noexcept;
+
+	bool SupportsContainerSuffix(const char *suffix) const noexcept {
+		return container_scan != nullptr && SupportsSuffix(suffix);
+	}
 };
 
 #endif