MINOR: lua: set skeleton for Lua execution expiration

This first patch permits to cofigure the Lua execution exipiration.
This expiration is configured but it is not yet avalaible, it will
be add in a future patch.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 5920e03..274659c 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -35,6 +35,7 @@
 	             -1 if the memory context is not used. */
 	int nargs; /* The number of arguments in the stack at the start of execution. */
 	unsigned int flags; /* The current execution flags. */
+	int expire; /* Lua execution must be stopped over this time. */
 	struct task *task; /* The task associated with the lua stack execution.
 	                      We must wake this task to continue the task execution */
 	struct list com; /* The list head of the signals attached to this task. */