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

Index: objects/button.c
--- objects/button.c.orig
+++ objects/button.c
@@ -35,6 +35,8 @@
 
 #include "button.h"
 
+lua_class_t button_class;
+
 /** Button object.
  *
  * @tfield int button The mouse button number, or 0 for any button.