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}
diff --git a/device/BUILD.gn b/device/BUILD.gn
index d72dd6f..194dabf 100644
--- a/device/BUILD.gn
+++ b/device/BUILD.gn
@@ -63,6 +63,7 @@
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/mojo/src/mojo/edk/system",
+    "//tools/usb_gadget",
     "//url",
   ]