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

Index: objects/drawin.h
Index: objects/key.c
--- objects/key.c.orig
+++ objects/key.c
@@ -43,6 +43,8 @@
 #include <xkbcommon/xkbcommon.h>
 #include <glib.h>
 
+lua_class_t key_class;
+
 /** Key object.
  *
  * @tfield string key The key to trigger an event.