x86: nmi and irq counters are unsigned in nmi_64.c

__nmi_count, apic_timer_irqs and irq0_irqs are unsigned.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>