platform/x86/intel-uncore-freq: make uncore_root_kobj static

Fix the following sparse warning:

drivers/platform/x86/intel-uncore-frequency.c:56:16: warning: symbol
'uncore_root_kobj' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>