Fix missing override keywords.

src/modes.cpp

@@ -303,7 +303,7 @@ void StackerInfo::AddMode(Mode *mode, bool set, const Anope::string &param)
 static class ModePipe : public Pipe
 {
  public:
-	void OnNotify()
+	void OnNotify() override
 	{
 		ModeManager::ProcessModes();
 	}