i386: fix build on pre 4.4 kernels

kexec build will fail on older kernels (pre 4.4) as the define
VIDEO_CAPABILITY_64BIT_BASE was not present at that time.

This patch adds it, as per linux/include/uapi/linux/screen_info.h,
if not present.

Signed-off-by: Federico Pellegrin <fede@evolware.org>
Reviewed-by: Kairui Song <kasong@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>