Core/Collision: fix no-PCH build

(cherry-picked from 65b6202bd8)

src/common/Collision/Models/WorldModel.cpp

@@ -19,6 +19,7 @@
 #include "WorldModel.h"
 #include "VMapDefinitions.h"
 #include "MapTree.h"
+#include "ModelInstance.h"
 #include "ModelIgnoreFlags.h"
 
 using G3D::Vector3;