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On all other platforms //device/usb uses a custom backend. This patch
ensures that we are not including libusb in unexpected build
configurations.

Change-Id: Ia1b68e86048af7ed98d27b19f46f74ceba50097b
Reviewed-on: https://chromium-review.googlesource.com/996407Reviewed-by: Jun Cai <juncai@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548282}