add override to bufferForScreen

Summary: This seems to just have been forgotten here.

Test Plan: Builds

Reviewers: graesslin

Reviewed By: graesslin

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1499

plugins/platforms/drm/scene_qpainter_drm_backend.h

@@ -37,7 +37,7 @@ public:
     virtual ~DrmQPainterBackend();
 
     QImage *buffer() override;
-    QImage *bufferForScreen(int screenId);
+    QImage *bufferForScreen(int screenId) override;
     bool needsFullRepaint() const override;
     bool usesOverlayWindow() const override;
     void prepareRenderingFrame() override;