x86, ioapic: Only export mp_find_ioapic and mp_find_ioapic_pin in io_apic.h

Multiple declarations of the same function in different headers
is a pain to maintain.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <1269936436-7039-6-git-send-email-ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>