x86, ioapic: Add dummy ioapic functions

Some ioapic extern functions are used when CONFIG_X86_IO_APIC is not
defined.  We need the dummy functions to avoid a compile time error.

Signed-off-by: Jacob Pan <jacob.jun.pan@intel.com>
LKML-Reference: <43F901BD926A4E43B106BF17856F0755A318DA07@orsmsx508.amr.corp.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>