Fix -fno-common build error. Taken from
https://gitlab.gnome.org/GNOME/atomix/-/commit/be7f44f1945a569494d46c60eaf6e7b39b2bb48b

Index: src/theme.c
--- src/theme.c.orig
+++ src/theme.c
@@ -32,7 +32,7 @@ static void theme_init (Theme *theme);
 static void theme_finalize (GObject *object);
 static void destroy_theme_image (gpointer data);
 
-GObjectClass *parent_class;
+static GObjectClass *parent_class;
 
 GType theme_get_type (void)
 {
