libcamera: framebuffer: Make FrameBuffer::cancel() private

FrameBuffer::cancel() is not meant to be used by applications. Move it
to the FrameBuffer::Private class.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
[Kieran: Handle rebase]
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/pipeline/raspberrypi/raspberrypi.cpp

@@ -1907,7 +1907,7 @@ void RPiCameraData::clearIncompleteRequests()
 			 * request? If not, do so now.
 			 */
 			if (buffer->request()) {
-				buffer->cancel();
+				buffer->_d()->cancel();
 				pipe()->completeBuffer(request, buffer);
 			}
 		}