x86: nmi.c fix style problems

Impact: cleanup, fix style problems

Fixes style problems:

 WARNING: Use #include <linux/smp.h> instead of <asm/smp.h>
 WARNING: Use #include <linux/nmi.h> instead of <asm/nmi.h>

total: 0 errors, 2 warnings

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>