x86: fix arch/x86/kernel/setup.c build warning when !CONFIG_X86_RESERVE_LOW_64K

Impact: cleanup

Fix:

  arch/x86/kernel/setup.c:592: warning: 'dmi_low_memory_corruption' defined but not used

this is only used if CONFIG_X86_RESERVE_LOW_64K is defined.

Signed-off-by: Richard A. Holden III <aciddeath@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>