Add a GN target to build the USB gadget Python package.

This GN target matches tools/usb_gadget/usb_gadget.gyp:usb_gadget.

BUG=None

Review URL: https://codereview.chromium.org/1671513002

Cr-Commit-Position: refs/heads/master@{#373628}
2 files changed