cpufreq: schedutil: Use to_gov_attr_set() to get the gov_attr_set

The to_gov_attr_set() has been moved to the cpufreq.h, so use it to get
the gov_attr_set.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>