driver core: cpu: fix section mismatch in cpu.c:store_online

driver core: cpu: fix section mismatch in cpu.c:store_online

Fix following warning:
WARNING: vmlinux.o(.text+0x64609c): Section mismatch in reference from the function store_online() to the function .cpuinit.text:cpu_up()

store_online() is defined inside a HOTPLUG_CPU block so references are OK.
Ignore references by annotating store_online() with __ref.

Note: This is needed because cpu_up() most likely should not have been
__cpuinit but all the hotplug cpu code misuses the __cpuinit annotation.

Signed-off-by: Sam Ravnborg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>