x86: smpboot.c: removed duplicated include

Removed duplicated include <asm/nmi.h> in
arch/x86/kernel/smpboot.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Cc: mingo@redhat.com
Cc: hpa@zytor.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>