trivial: Mark overrides as such.

This trivial change adds the "override" keyword to some methods of
subclasses meant to override interface methods.  This ensures that any
future change to the interface' method signatures which are not correctly
mirrored in the subclass will break at compile time with a clear error message,
rather than fail at runtime (which is harder to debug).

src/bench/bench.h

@@ -111,9 +111,9 @@ public:
 class ConsolePrinter : public Printer
 {
 public:
-    void header();
-    void result(const State& state);
-    void footer();
+    void header() override;
+    void result(const State& state) override;
+    void footer() override;
 };
 
 // creates box plot with plotly.js
@@ -121,9 +121,9 @@ class PlotlyPrinter : public Printer
 {
 public:
     PlotlyPrinter(std::string plotly_url, int64_t width, int64_t height);
-    void header();
-    void result(const State& state);
-    void footer();
+    void header() override;
+    void result(const State& state) override;
+    void footer() override;
 
 private:
     std::string m_plotly_url;