Need ifdef protect for extensions

[fnwinter]

for example, ObjectType::SceneObserverMSFT is protected by XR_MSFT_scene_understanding define.
but in openxr_handles_forward.hpp there is no protect attributes in traits.
it should be like this below.
#if defined(XR_MSFT_scene_understanding)
template <>
struct cpp_type_from_object_type_enumObjectType::SceneObserverMSFT {
using type = SceneObserverMSFT;
};
#endif // defined(XR_MSFT_scene_understanding)

[fnwinter]

thank you for merging and fixing it.