uefi: Don't override _FORTIFY_SOURCE when building the EFI binary
uefi: Don't override _FORTIFY_SOURCE when building the EFI binary

Fixes https://github.com/hughsie/fwupd/issues/631

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