Convert pci/ioapic.c to be builtin only, with no module option, so we can
support IO-APIC hotplug.  Also make it depend on X86_IO_APIC.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>