includecheck fix: x86, cpu/common.c

fix the following 'make includecheck' warning:

  arch/x86/kernel/cpu/common.c: linux/smp.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1252087783.6385.10.camel@ht.satnam>