Fix -Winconsistent-missing-destructor-override

Change-Id: I7a33d6b0cf0bd775567235e0efc858796d8ad852
Reviewed-by: David Faure <david.faure@kdab.com>

src/widgets/util/qcompleter.h

@@ -87,7 +87,7 @@ public:
 #if QT_CONFIG(stringlistmodel)
     QCompleter(const QStringList& completions, QObject *parent = nullptr);
 #endif
-    ~QCompleter();
+    ~QCompleter() override;
 
     void setWidget(QWidget *widget);
     QWidget *widget() const;