x86: uniq_ioapic_id - fix section mismatch warning

Fix folowing warning:
WARNING: arch/x86/kernel/built-in.o(.text+0x10799): Section mismatch in reference from the function uniq_ioapic_id()

uniq_ioapic_id() is only used by __init mp_register_ioapic(). Annotate uniq_ioapic_id() with __init.

Signed-off-by: Jacek Luczak <luczak.jacek@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>