x86: Add dummy functions for compiling without IOAPIC

This patch adds IOAPIC dummy functions for compilation
with local APIC, but without IOAPIC.

The local variable ioapic_entries in enable_IR_x2apic()
does not need initialization anymore, since the dummy
returns NULL.

Signed-off-by: Henrik Kretzschmar <henne@nachtwindheim.de>
LKML-Reference: <1298385487-4708-4-git-send-email-henne@nachtwindheim.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>