x86: apic.c x2apic_preenabled and disable_x2apic should be static

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: Jaswinder Singh <jaswinder@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>