compilation of `ftinspect` fails

[Werner LEMBERG]

[branch gsoc-2022-chariri-2, commit 6a264517f2]

Compilation of 'freetype2-demos' using

```
meson setup meson-build
meson compile -C meson-build
```

fails with

```
../src/ftinspect/widgets/tripletselector.cpp: In member function ‘void 
TripletSelector::repopulateFonts()’:
../src/ftinspect/widgets/tripletselector.cpp:46:16: error: ‘class QFileInfo’ 
has no member named ‘isSymbolicLink’; did you mean ‘isSymLink’?
       if (info.isSymbolicLink())
                ^~~~~~~~~~~~~~
                isSymLink
../src/ftinspect/widgets/tripletselector.cpp: In member function ‘void 
TripletSelector::createLayout()’:
../src/ftinspect/widgets/tripletselector.cpp:324:18: error: ‘class QComboBox’ 
has no member named ‘setPlaceholderText’
   fontComboBox_->setPlaceholderText(tr("No font open"));
                  ^~~~~~~~~~~~~~~~~~
../src/ftinspect/widgets/tripletselector.cpp:325:18: error: ‘class QComboBox’ 
has no member named ‘setPlaceholderText’
   faceComboBox_->setPlaceholderText(tr("No face available"));
                  ^~~~~~~~~~~~~~~~~~
../src/ftinspect/widgets/tripletselector.cpp:326:16: error: ‘class QComboBox’ 
has no member named ‘setPlaceholderText’
   niComboBox_->setPlaceholderText(tr("No named instance available"));
                ^~~~~~~~~~~~~~~~~~
```


    Werner