Add missing override keyword found by Clang

qbsdfbscreen.h:57:10: error: 'initialize' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

Change-Id: I117816bf0f5e469b8d34fffd153e649318d2fd7c
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>

src/plugins/platforms/bsdfb/qbsdfbscreen.h

@@ -54,7 +54,7 @@ public:
     explicit QBsdFbScreen(const QStringList &args);
     ~QBsdFbScreen() override;
 
-    bool initialize();
+    bool initialize() override;
 
     QPixmap grabWindow(WId wid, int x, int y, int width, int height) const override;