Use override instead of virtual for subclasses.

Now that we are using c++11, it is safer to use override in subclasses
instead of virtual.

This CL does it for the interface, a follow up CL will do it for
other parts of the code base.

Change-Id: Ice8d0f4355cb700019d7a9c1566fbff0099e97d6

include/ceres/numeric_diff_cost_function.h

@@ -196,9 +196,9 @@ class NumericDiffCostFunction : public SizedCostFunction<kNumResiduals, Ns...> {
     }
   }
 
-  virtual bool Evaluate(double const* const* parameters,
-                        double* residuals,
-                        double** jacobians) const {
+  bool Evaluate(double const* const* parameters,
+                double* residuals,
+                double** jacobians) const override {
     using internal::FixedArray;
     using internal::NumericDiff;