cpufreq: Mark function as static in cpufreq.c

Mark function as static in cpufreq.c because it is not
used outside this file.

This eliminates the following warning in cpufreq.c:
drivers/cpufreq/cpufreq.c:355:9: warning: no previous prototype for ‘show_boost’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/cpufreq.c

@@ -352,7 +352,7 @@ EXPORT_SYMBOL_GPL(cpufreq_notify_post_transition);
 /*********************************************************************
  *                          SYSFS INTERFACE                          *
  *********************************************************************/
-ssize_t show_boost(struct kobject *kobj,
+static ssize_t show_boost(struct kobject *kobj,
 				 struct attribute *attr, char *buf)
 {
 	return sprintf(buf, "%d\n", cpufreq_driver->boost_enabled);