Enforce 4 sec Sneak Attack rule fixes #569

Spells flagged with SNEAK_ATTACK requires you to be hidden for
about 4 seconds before you can cast them

zone/mob.h

@@ -762,6 +762,7 @@ public:
 	inline const bodyType GetOrigBodyType() const { return orig_bodytype; }
 	void SetBodyType(bodyType new_body, bool overwrite_orig);
 
+	uint32 tmHidden; // timestamp of hide, only valid while hidden == true
 	uint8 invisible, see_invis;
 	bool invulnerable, invisible_undead, invisible_animals, sneaking, hidden, improved_hidden;
 	bool see_invis_undead, see_hide, see_improved_hide;