Implement Precompiled Headers (#591)

When applied, this will speed up compile times dramatically.

dGame/dInventory/Inventory.h

@@ -1,5 +1,8 @@
 #pragma once
 
+#ifndef INVENTORY_H
+#define INVENTORY_H
+
 #include <map>
 #include <vector>
 
@@ -182,3 +185,5 @@ private:
      */
 	static std::vector<LOT> m_GameMasterRestrictedItems;
 };
+
+#endif