Fixes build with -fno-common

diff -Nurp atomix-3.34.0/src/level.c atomix-3.34.0-fix/src/level.c
--- atomix-3.34.0/src/level.c	2019-09-09 19:14:36.000000000 -0000
+++ atomix-3.34.0-fix/src/level.c	2020-09-12 23:21:04.917492187 -0000
@@ -31,7 +31,7 @@ static void level_class_init (GObjectCla
 static void level_init (Level *level);
 static void level_finalize (GObject *object);
 
-GObjectClass *parent_class;
+static GObjectClass *parent_class;
 
 /*=================================================================
  
diff -Nurp atomix-3.34.0/src/theme.c atomix-3.34.0-fix/src/theme.c
--- atomix-3.34.0/src/theme.c	2019-09-09 19:14:36.000000000 -0000
+++ atomix-3.34.0-fix/src/theme.c	2020-09-12 23:21:19.557735891 -0000
@@ -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)
 {