Fix -Wmismatched-tags warnings

Fixes all occurences of this warning:
warning: '<type>' defined as a struct here but previously declared as a class
      [-Wmismatched-tags]

rwengine/src/engine/GameState.hpp

@@ -243,8 +243,9 @@ struct GarageInfo
  * Gameplay state object that holds persistent state, and references runtime
  * world state.
  */
-struct GameState
+class GameState
 {
+public:
 	/**
 	  Basic Game State
 	 */

rwengine/src/objects/VehicleObject.hpp

@@ -6,7 +6,7 @@
 #include <objects/VehicleInfo.hpp>
 
 class CollisionInstance;
-class btVehicleRaycaster;
+struct btVehicleRaycaster;
 class btRaycastVehicle;
 class btRigidBody;
 class btHingeConstraint;

rwengine/src/render/GameRenderer.hpp

@@ -21,7 +21,7 @@ class ModelFrame;
 class GameWorld;
 class GameObject;
 
-class AreaIndicatorInfo;
+struct AreaIndicatorInfo;
 
 /// @todo migrate to some other way of rendering each object type.
 class CharacterObject;