configure_system: Make public slots private

These are only used within this class, so we can make them private to
keep their use contained. This also gets rid of the pre-Qt5 'slot'
identifier, since Qt 5's connection syntax doesn't require a function to
be declared a slot anymore.

src/citra_qt/configuration/configure_system.h

@@ -29,15 +29,14 @@ public:
     void setConfiguration();
     void retranslateUi();
 
-public slots:
-    void updateBirthdayComboBox(int birthmonth_index);
-    void updateInitTime(int init_clock);
-    void refreshConsoleID();
-
 private:
     void ReadSystemSettings();
     void ConfigureTime();
 
+    void updateBirthdayComboBox(int birthmonth_index);
+    void updateInitTime(int init_clock);
+    void refreshConsoleID();
+
     std::unique_ptr<Ui::ConfigureSystem> ui;
     bool enabled;