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

Index: objects/client.c
--- objects/client.c.orig
+++ objects/client.c
@@ -106,6 +106,8 @@
 #include <xcb/shape.h>
 #include <cairo-xcb.h>
 
+lua_class_t client_class;
+
 /** Client class.
  *
  * This table allow to add more dynamic properties to the clients. For example,