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

Index: src/level.c
--- src/level.c.orig
+++ src/level.c
@@ -31,7 +31,7 @@ static void level_class_init (GObjectClass *class);
 static void level_init (Level *level);
 static void level_finalize (GObject *object);
 
-GObjectClass *parent_class;
+static GObjectClass *parent_class;
 
 /*=================================================================
  
