efi: gsmi: fix false dependency on CONFIG_EFI_VARS

The gsmi code does not actually rely on CONFIG_EFI_VARS, since it only
uses the efivars abstraction that is included unconditionally when
CONFIG_EFI is defined. CONFIG_EFI_VARS controls the inclusion of the
code that exposes the sysfs entries, and which has been deprecated for
some time.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>