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: Ingo Molnar <mingo@elte.hu>