trivial: uefi: update the comment for _efi_get_variable_exists
trivial: uefi: update the comment for _efi_get_variable_exists

The commit was merged in https://github.com/rhboot/efivar/commit/0b3d29d2d850f9777ac2343346172569673b9f7b
which landed in efivar 35.

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