x86, nmi: fix build

fix:

arch/x86/kernel/built-in.o: In function `proc_nmi_enabled':
: undefined reference to `nmi_watchdog_default'
arch/x86/kernel/built-in.o: In function `native_smp_prepare_cpus':
: undefined reference to `nmi_watchdog_default'

Signed-off-by: Ingo Molnar <mingo@elte.hu>