libgusb: Drop strict requirement for udev-enabled libusb

It's also usable without

BUG=b:177372789
TEST=with more changes around fwupd, fizz-labstation has fwupd built
with genesys plugin, which requires gusb, even though udev is disabled
for labstation.

Change-Id: I600d5eb184850aa3727cf802ffd6fbb6228bd502
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3736322
Commit-Queue: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Tested-by: Patrick Georgi <pgeorgi@chromium.org>
2 files changed