clocksource, acpi_pm.c: put acpi_pm_read_slow() under CONFIG_PCI

acpi_pm_read_slow() is only used when CONFIG_PCI=y, so move the definition
inside the ifdef.

Otherwise this causes a "defined but not used" warning when building with
CONFIG_ACPI=y and CONFIG_PCI=n (that's not supported yet, but it could
be).

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: mingo@elte.hu
Cc: johnstul@us.ibm.com
Cc: akpm@linux-foundation.org
Cc: bjorn.helgaas@hp.com
Cc: lenb@kernel.org
Cc: linux@dominikbrodowski.net
Cc: tglx@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>