decoder_list: convert to C++

src/decoder_plugin.h

@@ -190,6 +190,10 @@ decoder_plugin_container_scan(	const struct decoder_plugin *plugin,
 	return plugin->container_scan(pathname, tnum);
 }
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Does the plugin announce the specified file name suffix?
  */
@@ -204,4 +208,8 @@ bool
 decoder_plugin_supports_mime_type(const struct decoder_plugin *plugin,
 				  const char *mime_type);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif