Fix build with llvm 22

Index: libgiggle/giggle-plugin.c
--- libgiggle/giggle-plugin.c.orig
+++ libgiggle/giggle-plugin.c
@@ -517,7 +517,7 @@ giggle_plugin_new_from_file (const char  *filename,
 	object = gtk_builder_get_object (builder, "plugin");
 
 	if (object) {
-		plugin = g_object_ref (object);
+		plugin = GIGGLE_PLUGIN(g_object_ref (object));
 		giggle_plugin_set_builder (plugin, builder);
 		giggle_plugin_set_filename (plugin, filename);
 	} else {
