Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:

  arch/x86/kernel/apic.c:103:5: warning: symbol 'disable_x2apic' was not declared. Should it be static?
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>