platform/x86: intel_pmc_core: Mark local function static

0day reports:

drivers/platform/x86/intel_pmc_core.c:833:5: sparse:
	symbol 'quirk_xtal_ignore' was not declared. Should it be static?

Mark the function static since it is indeed only called locally.

Cc: Rajneesh Bhardwaj <rajneesh.bhardwaj@linux.intel.com>
Fixes: 238f9c11 ("platform/x86: intel_pmc_core: Quirk to ignore XTAL shutdown")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>