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

Fixes sparse warning:

  arch/x86/kernel/io_apic.c:709:6: warning: symbol 'io_apic_sync' was not declared. Should it be static?
Signed-off-by: NJaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>