fix(build): fix DlgSettingsGeneral::applyMenuIconSize visibility and namespace

- Move applyMenuIconSize to public access so StartupProcess can call it
- Add Dialog:: namespace qualifier in StartupProcess.cpp

src/Gui/PreferencePages/DlgSettingsGeneral.h

@@ -93,6 +93,8 @@ private:
     int getCurrentMenuIconSize() const;
     void addMenuIconSizes(int current);
     void translateMenuIconSizes();
+
+public:
     static void applyMenuIconSize(int pixel);
 
 private:

src/Gui/StartupProcess.cpp

@@ -288,7 +288,7 @@ void StartupPostProcess::setMenuIconSize()
     ParameterGrp::handle hGrp = WindowParameter::getDefaultParameter()->GetGroup("General");
     int size = int(hGrp->GetInt("MenuIconSize", 0));
     if (size >= 16) {
-        DlgSettingsGeneral::applyMenuIconSize(size);
+        Dialog::DlgSettingsGeneral::applyMenuIconSize(size);
     }
 }