x86: cpu_index build fix

fix:

 arch/x86/kernel/cpu/common.c: In function 'early_identify_cpu':
 arch/x86/kernel/cpu/common.c:553: error: 'struct cpuinfo_x86' has no member named 'cpu_index'

as cpu_index is only available on SMP.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/common.c

@@ -550,7 +550,9 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	validate_pat_support(c);
 
+#ifdef CONFIG_SMP
 	c->cpu_index = boot_cpu_id;
+#endif
 }
 
 void __init early_cpu_init(void)