Defined settingsChanged() signal for UsageStatisticPage

ui/usagestatisticpage.h

@@ -51,6 +51,9 @@ public: // IOptionsPage interface
     void apply() override;
     void finish() override;
 
+signals:
+    void settingsChanged();
+
 private: // Data
     std::unique_ptr<KUserFeedback::FeedbackConfigWidget> m_feedbackWidget;
     std::shared_ptr<KUserFeedback::Provider> m_provider;