Prevent overriding virtual methods that are called in a destructor

Virtual methods are no longer valid once the derived destructor has
run. This means the compiler is free to optimize them to be
non-virtual.

Found using clang-tidy

src/libmain/progress-bar.cc

@@ -108,7 +108,8 @@ public:
         stop();
     }
 
-    void stop() override
+    /* Called by destructor, can't be overridden */
+    void stop() override final
     {
         {
             auto state(state_.lock());