fix: add missing override keyword

Change-Id: I222e33a92ffcb00a667d6224961320b46a1ec8ba
Reviewed-on: https://codereview.kdab.com/c/knut/+/141867
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Tested-by: Continuous Integration <build@kdab.com>

src/core/imagedocument.h

@@ -11,7 +11,7 @@ class ImageDocument : public Document
     Q_OBJECT
 
 public:
-    ImageDocument(QObject *parent = nullptr);
+    explicit ImageDocument(QObject *parent = nullptr);
 
     QImage image() const;