libv4lplugins: Add an empty libv4l-rockchip_v2 subdirectory.

BUG=chrome-os-partner:55135
TEST=Build on kevin and veyron_minnie

Change-Id: I3648ecac1be86eac382d6693be8891074335b07c
Reviewed-on: https://chromium-review.googlesource.com/368553


Commit-Ready: Owen Lin <owenlin@chromium.org>
Tested-by: Owen Lin <owenlin@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>

libv4l-rockchip_v2/Makefile.am

+# Copyright 2016 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+