Bugfix: Support for JSONcpp >= 1.7.7
As pointed out in #16938 (closed), we need to cast vtkIdType to int64_t, if compiling with VTK_USE_64BIT_IDS=ON.
As pointed out in #16938 (closed), we need to cast vtkIdType to int64_t, if compiling with VTK_USE_64BIT_IDS=ON.