ACPI / PM: Fix build warning in sleep.c for CONFIG_ACPI_SLEEP unset

If CONFIG_ACPI_SLEEP is unset, the compiler complains that
pwr_btn_event_pending is defined but not used.  To silence the
warning, move the definition of pwr_btn_event_pending under an
appropriate #ifdef.

Reported-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Len Brown <len.brown@intel.com>