Fixed forward declaration of struct Slic3r::UndoRedo::Snapshot so MSVC does not complain
Fixed forward declaration of struct Slic3r::UndoRedo::Snapshot so MSVC does not complain

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r