igb_uio: fix build with lock down checks

igb_uio: fix build with lock down checks

Caught on ubuntu-16.04 with hwe kernel for aarch64:

$ uname -a
Linux ubuntu1604arm64es 4.13.0-43-generic #48~16.04.1-Ubuntu SMP Thu May
17 13:08:01 UTC 2018 aarch64 aarch64 aarch64 GNU/Linux

== Build kernel/linux/igb_uio
  CC [M] .../kernel/linux/igb_uio/igb_uio.o
In file included from .../kernel/linux/igb_uio/igb_uio.c:20:0:
.../igb_uio/compat.h: In function ‘igbuio_kernel_is_locked_down’:
.../igb_uio/compat.h:146:7:
error: "CONFIG_EFI_SECURE_BOOT_LOCK_DOWN" is not defined [-Werror=undef]
 #elif CONFIG_EFI_SECURE_BOOT_LOCK_DOWN
       ^
cc1: all warnings being treated as errors

Fixes: d67014c3d38b ("igb_uio: fail and log if kernel lock down is enabled")

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>