ACPI: PCC: pcc_ctx can be static

drivers/acpi/acpi_pcc.c:34:22: warning: symbol 'pcc_ctx' was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>