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

include/ceres/dynamic_numeric_diff_cost_function.h

@@ -89,7 +89,7 @@ class DynamicNumericDiffCostFunction : public DynamicCostFunction {
       DynamicNumericDiffCostFunction&& other)
       : functor_(std::move(other.functor_)), ownership_(other.ownership_) {}
 
-  virtual ~DynamicNumericDiffCostFunction() {
+  ~DynamicNumericDiffCostFunction() override {
     if (ownership_ != TAKE_OWNERSHIP) {
       functor_.release();
     }