Silence Clang warning

Clang emits the following warning:

internal/ceres/schur_complement_solver.cc:71:51: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  virtual ~BlockRandomAccessSparseMatrixAdapter() final {}

Change-Id: I9da96711fd1e924ce8504988ff171cb476b6222f
diff --git a/internal/ceres/schur_complement_solver.cc b/internal/ceres/schur_complement_solver.cc
index 5c81c33..4836981 100644
--- a/internal/ceres/schur_complement_solver.cc
+++ b/internal/ceres/schur_complement_solver.cc
@@ -61,14 +61,14 @@
 
 namespace {
 
-class BlockRandomAccessSparseMatrixAdapter
+class BlockRandomAccessSparseMatrixAdapter final
     : public ConjugateGradientsLinearOperator<Vector> {
  public:
   explicit BlockRandomAccessSparseMatrixAdapter(
       const BlockRandomAccessSparseMatrix& m)
       : m_(m) {}
 
-  virtual ~BlockRandomAccessSparseMatrixAdapter() final {}
+  virtual ~BlockRandomAccessSparseMatrixAdapter() {}
 
   void RightMultiplyAndAccumulate(const Vector& x, Vector& y) final {
     m_.SymmetricRightMultiplyAndAccumulate(x.data(), y.data());