trivial: don't include gusb in pkg-config unless gusb enabled
trivial: don't include gusb in pkg-config unless gusb enabled

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