thinkpad-acpi: make drivers/misc/thinkpad_acpi:fan_mutex static

This patch makes the needlessly global fan_mutex static.
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/misc/thinkpad_acpi.h

@@ -386,7 +386,7 @@ static u8 fan_control_initial_status;
 static u8 fan_control_desired_level;
 static int fan_watchdog_maxinterval;
 
-struct mutex fan_mutex;
+static struct mutex fan_mutex;
 
 static acpi_handle fans_handle, gfan_handle, sfan_handle;