Tolerate annoying webcams which return undocumented errors

Change-Id: I7797178132103f43af3ff9c4cc20d2de6bd9c908
diff --git a/y2020/vision/v4l2_reader.h b/y2020/vision/v4l2_reader.h
index 969f4a8..bdf4a8e 100644
--- a/y2020/vision/v4l2_reader.h
+++ b/y2020/vision/v4l2_reader.h
@@ -88,6 +88,8 @@
 
   int Ioctl(unsigned long number, void *arg);
 
+  void StreamOff();
+
   // The mmaped V4L2 buffers.
   std::array<Buffer, kNumberBuffers> buffers_;