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: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>
Tested-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NLen Brown <len.brown@intel.com>

drivers/acpi/sleep.c

@@ -57,7 +57,6 @@ MODULE_PARM_DESC(gts, "Enable evaluation of _GTS on suspend.");
 MODULE_PARM_DESC(bfs, "Enable evaluation of _BFS on resume".);
 
 static u8 sleep_states[ACPI_S_STATE_COUNT];
-static bool pwr_btn_event_pending;
 
 static void acpi_sleep_tts_switch(u32 acpi_state)
 {
@@ -110,6 +109,7 @@ static int acpi_sleep_prepare(u32 acpi_state)
 
 #ifdef CONFIG_ACPI_SLEEP
 static u32 acpi_target_sleep_state = ACPI_STATE_S0;
+static bool pwr_btn_event_pending;
 
 /*
  * The ACPI specification wants us to save NVS memory regions during hibernation