message: Fix include issues when compiling with sundials
diff: #define MFEM_SOLVERS
  #include "../config/config.hpp"
  -#include "operator.hpp"
  +#include "densemat.hpp"
  #ifdef MFEM_USE_MPI
  #include <mpi.h>
  @@ -542,7 +542,7 @@ public:
  private:
  /// Set up the block CSR structure corresponding to a sparse matrix @a A
  - void CreateBlockPattern(const SparseMatrix &A);
  + void CreateBlockPattern(const class SparseMatrix &A);
  /// Perform the block ILU factorization
  void Factorize();
  