trivial: Fix a Werror=sign-compare on Fedora 28
trivial: Fix a Werror=sign-compare on Fedora 28

vathpela/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,vathpela/fwupd,superm1/fwupd,fwupd/fwupd,fwupd/fwupd,superm1/fwupd,hughsie/fwupd,hughsie/fwupd,superm1/fwupd,vathpela/fwupd,vathpela/fwupd,hughsie/fwupd