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>

drivers/platform/x86/intel-uncore-frequency.c

@@ -53,7 +53,7 @@ static int uncore_max_entries __read_mostly;
 /* Storage for uncore data for all instances */
 static struct uncore_data *uncore_instances;
 /* Root of the all uncore sysfs kobjs */
-struct kobject *uncore_root_kobj;
+static struct kobject *uncore_root_kobj;
 /* Stores the CPU mask of the target CPUs to use during uncore read/write */
 static cpumask_t uncore_cpu_mask;
 /* CPU online callback register instance */