The function acpi_processor_check_duplicates() is local to the source
and does not need to be in global scope, so make it static.

Cleans up sparse warnings:
symbol 'acpi_processor_check_duplicates' was not declared. Should it
be static?
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>