trivial: Fix tiny build warning with GCC6
trivial: Fix tiny build warning with GCC6

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