Fact: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

src/FactSystem/Fact.cc

@@ -737,7 +737,7 @@ FactValueSliderListModel* Fact::valueSliderModel(void)
     return _valueSliderModel;
 }
 
-void Fact::_checkForRebootMessaging(void)
+void Fact::_checkForRebootMessaging(void) const
 {
     if(qgcApp()) {
         if (!qgcApp()->runningUnitTests()) {

src/FactSystem/Fact.h

@@ -191,7 +191,7 @@ signals:
     void _containerRawValueChanged(const QVariant& value);
 
 private slots:
-    void _checkForRebootMessaging(void);
+    void _checkForRebootMessaging(void) const;
 
 private:
     void _init(void);