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/request.cpp

@@ -129,7 +129,7 @@ void Request::Private::doCancelRequest()
 	Request *request = _o<Request>();
 
 	for (FrameBuffer *buffer : pending_) {
-		buffer->cancel();
+		buffer->_d()->cancel();
 		camera_->bufferCompleted.emit(request, buffer);
 	}