cpufreq: governor: Make gov_set_update_util() static

The gov_set_update_util() routine is only used internally by the
common governor code and it doesn't need to be exported, so make
it static.

No functional changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>