x86: fix section mismatch warning in acpi/boot.c

Fix following warning:
WARNING: o-x86_64/arch/x86/kernel/built-in.o(.text+0x13d15): Section mismatch in reference from the function acpi_map_lsapic() to the function .cpuinit.text:mp_register_lapic()

The function acpi_map_lsapic() is exported and thus not annotated.
But the sole user is acpi/processor_core.c in a __cpuinit path.
So create a small wrapper and put back the annotation thus
avoiding the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>