x86: mpparse.c: fix section mismatch warning

WARNING: vmlinux.o(.text+0x118f7): Section mismatch in reference from the function construct_ioapic_table() to the function .init.text:MP_bus_info()
The function construct_ioapic_table() references
the function __init MP_bus_info().
This is often because construct_ioapic_table lacks a __init
annotation or the annotation of MP_bus_info is wrong.

construct_ioapic_table is called only from construct_default_ISA_mptable which is __init

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>