ACPI: PM: s2idle: fix section mismatch warning

The acpi_sleep_suspend_setup() function is missing an __init annotation,
which causes a warning in rare configurations that end up not inlining
it into its caller:

WARNING: modpost: vmlinux.o: section mismatch in reference: acpi_sleep_suspend_setup (section: .text) -> acpi_s2idle_setup (section: .init.text)

It's only called from an __init function, so adding the annotation is
correct here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>