thermal: cpu_cooling: Make 'notify_device' static

Silences the following sparse warning:

drivers/thermal/cpu_cooling.c:67:31: warning:
symbol 'notify_device' was not declared. Should it be static?
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/cpu_cooling.c

@@ -64,7 +64,7 @@ static unsigned int cpufreq_dev_count;
 
 /* notify_table passes value to the CPUFREQ_ADJUST callback function. */
 #define NOTIFY_INVALID NULL
-struct cpufreq_cooling_device *notify_device;
+static struct cpufreq_cooling_device *notify_device;
 
 /**
  * get_idr - function to get a unique id.