Common/DebugInterface: Default virtual destructor

While we're at it, we can also default the constructor and destructor of
inheriting classes in their respective cpp file to prevent the
construction and destruction of non-trivial types being inlined into
other regions of code.

Source/Core/Core/Debugger/PPCDebugInterface.h

@@ -20,7 +20,9 @@ private:
 class PPCDebugInterface final : public Common::DebugInterface
 {
 public:
-  PPCDebugInterface() {}
+  PPCDebugInterface();
+  ~PPCDebugInterface() override;
+
   // Watches
   std::size_t SetWatch(u32 address, std::string name = "") override;
   const Common::Debug::Watch& GetWatch(std::size_t index) const override;