Impact: reduce kernel size a bit, address sparse warning

Addresses the problem pointed out by this sparse warning:
  arch/x86/kernel/apic/x2apic_cluster.c:13:1: warning: symbol 'per_cpu__x86_cpu_to_logical_apicid' was not declared. Should it be static?
Signed-off-by: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1239434726.4418.24.camel@localhost.localdomain>
Signed-off-by: NIngo Molnar <mingo@elte.hu>