trivial: fix gir compilation when gusb is turned off
trivial: fix gir compilation when gusb is turned off

hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd