encoder_list: add macro _for_each()

src/encoder_list.h

@@ -24,6 +24,14 @@
 
 struct encoder_plugin;
 
+extern const struct encoder_plugin *const encoder_plugins[];
+
+#define encoder_plugins_for_each(plugin) \
+	for (const struct encoder_plugin *plugin, \
+		*const*encoder_plugin_iterator = &encoder_plugins[0]; \
+		(plugin = *encoder_plugin_iterator) != NULL; \
+		++encoder_plugin_iterator)
+
 /**
  * Looks up an encoder plugin by its name.
  *