register_memory() becomes double definition in 2.6.20-rc1. It is defined
in arch/i386/kernel/setup.c as static definition in 2.6.19. But it is
moved to arch/i386/kernel/e820.c in 2.6.20-rc1. And same name function is
defined in driver/base/memory.c too. So, it becomes cause of compile error
of duplicate definition if memory hotplug option is on.