message: try to fix MSVC error
diff: @@ -34,6 +34,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  #define GKO_CORE_SOLVER_MULTIGRID_HPP_
  +#include <functional>
  #include <memory>
  #include <utility>
  @@ -101,7 +102,8 @@ public:
  std::vector<std::shared_ptr<const stop::CriterionFactory>>
  GKO_FACTORY_PARAMETER(criteria, nullptr);
  - std::vector<std::shared_ptr<const multigrid::RestrictProlongFactory>>
  + std::vector<
  + std::shared_ptr<const gko::multigrid::RestrictProlongFactory>>
  GKO_FACTORY_PARAMETER(rstr_prlg, nullptr);
  std::function<size_type(const size_type, const size_type)>
  @@ -204,7 +206,8 @@ protected:
  private:
  std::shared_ptr<const LinOp> system_matrix_{};
  std::shared_ptr<const stop::CriterionFactory> stop_criterion_factory_{};
  - std::vector<std::shared_ptr<multigrid::RestrictProlong>> rstr_prlg_list_{};
  + std::vector<std::shared_ptr<gko::multigrid::RestrictProlong>>
  + rstr_prlg_list_{};
  std::vector<std::shared_ptr<LinOp>> pre_smoother_list_{};
  std::vector<std::shared_ptr<LinOp>> post_smoother_list_{};
  std::shared_ptr<LinOp> coarsest_solver_{};
  