Fix -fno-common build failure. Taken from
https://github.com/awesomeWM/awesome/commit/d256d9055095f27a33696e0aeda4ee20ed4fb1a0

Index: objects/window.c
--- objects/window.c.orig
+++ objects/window.c
@@ -59,6 +59,7 @@
 #include "property.h"
 #include "xwindow.h"
 
+lua_class_t window_class;
 LUA_CLASS_FUNCS(window, window_class)
 
 static xcb_window_t