acpi: fix section mismatch warning in pnpacpi

Fix following section mismatch warning:
WARNING: vmlinux.o(.text+0x153d69): Section mismatch in reference from the function is_exclusive_device() to the variable .init.data:excluded_id_list

is_exclusive_device is only used from __init context so document
this with the __init annotation and get rid of the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Len Brown <len.brown@intel.com>