[trace] Fix destructor declaration

The destructor must be defined in the implementation class so that it
can be called, as Vedant Kumar pointed out in:

'''
What were your thoughts, re:

+class Trace : public PluginInterface {
+public:
+  ~Trace() override = default;

   Does this need to be `virtual ~Trace() = ...`?

      Otherwise, when a std::shared_ptr<Trace> is destroyed, the
      destructor for the derived TraceIntelPT instance won't run.
'''