Fix warning in nmi.c on uniprocessor kernels

Fix 

  CC      arch/x86_64/kernel/nmi.o
linux/arch/x86_64/kernel/nmi.c: In function ‘check_nmi_watchdog’:
linux/arch/x86_64/kernel/nmi.c:155: warning: statement with no effect

on Uniprocessor builds.

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/kernel/nmi.c
===================================================================
--- linux.orig/arch/x86_64/kernel/nmi.c
+++ linux/arch/x86_64/kernel/nmi.c
@@ -151,8 +151,10 @@ int __init check_nmi_watchdog (void)
 
 	printk(KERN_INFO "testing NMI watchdog ... ");
 
+#ifdef CONFIG_SMP
 	if (nmi_watchdog == NMI_LOCAL_APIC)
 		smp_call_function(nmi_cpu_busy, (void *)&endflag, 0, 0);
+#endif
 
 	for (cpu = 0; cpu < NR_CPUS; cpu++)
 		counts[cpu] = cpu_pda(cpu)->__nmi_count;