KVM: selftests: Add CONFIG_64BIT definition for the build
Since only 64bit KVM selftests were supported on all architectures,
add the CONFIG_64BIT definition in kvm/Makefile to ensure only 64bit
definitions were available in the corresponding included files.
Suggested-by: Andrew Jones <ajones@ventanamicro.com> Signed-off-by: Haibo Xu <haibo1.xu@intel.com> Reviewed-by: Andrew Jones <ajones@ventanamicro.com> Signed-off-by: Anup Patel <anup@brainfault.org>