x86: Fix false positive section mismatch in es7000_32.c

The variable apic_es7000_cluster references the function __cpuinit
wakeup_secondary_cpu_via_mip() from a noninit section. So we've been
warned by the following warning. To avoid possible collision between
init/noninit, its best to mark the variable as __refdata.

We were warned by the following warning:

  LD      arch/x86/kernel/apic/built-in.o
  WARNING: arch/x86/kernel/apic/built-in.o(.data+0x198c): Section
  mismatch in reference from the variable apic_es7000_cluster to the
  function .cpuinit.text:wakeup_secondary_cpu_via_mip()

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
LKML-Reference: <b9df5fa10907120404k6279a10ch5e9682432272706f@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>