Windows build fix

The MSVC linker is really pedantic with this, functions compiled as taking a class can't be linked with functions giving a struct, because of the name mangling I'm guessing...

```unresolved external symbol "protected: void __thiscall MWWorld::LiveCellRefBase::loadImp(class ESM::ObjectState const &)" (?loadImp@LiveCellRefBase@MWWorld@@IAEXABVObjectState@ESM@@@Z)```