x86, mmiotrace: silence section mismatch warning - leave_uniprocessor

WARNING: vmlinux.o(.text+0x180af): Section mismatch in reference from the function leave_uniprocessor() to the function .cpuinit.text:cpu_up()
The function leave_uniprocessor() references
the function __cpuinit cpu_up().
This is often because leave_uniprocessor lacks a __cpuinit
annotation or the annotation of cpu_up is wrong.

leave_uniprocessor calls cpu_up only when CONFIG_HOTPLUG_CPU is set,
so it can be safely annotated as __ref

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pekka Paalanen <pq@iki.fi>