efi_loader: SPDX-License-Identifier for efi.h

Add the missing SPDX-License-Identifier based on Linux kernel commit
b24413180f56 ("License cleanup: add SPDX GPL-2.0 license identifier to
files with no license").

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>