Replace virtual keyword by override

virtual can be ambiguous. Applied changes correspond to clang-tidy fixes
stemming from the modernize-use-override check.

Change-Id: I973afd4680a5df587419777504aeb94467196b89
diff --git a/internal/ceres/suitesparse.h b/internal/ceres/suitesparse.h
index 23f539d..a98c946 100644
--- a/internal/ceres/suitesparse.h
+++ b/internal/ceres/suitesparse.h
@@ -293,7 +293,7 @@
   static std::unique_ptr<SparseCholesky> Create(OrderingType ordering_type);
 
   // SparseCholesky interface.
-  virtual ~SuiteSparseCholesky();
+  ~SuiteSparseCholesky() override;
   CompressedRowSparseMatrix::StorageType StorageType() const final;
   LinearSolverTerminationType Factorize(CompressedRowSparseMatrix* lhs,
                                         std::string* message) final;