cpufreq: intel_pstate: Do not include debugfs.h

The intel_pstate driver doesn't use debugfs any more, so drop
linux/debugfs.h from the list of included headers in it.

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