platform/x86: thinkpad_acpi: Make some symbols static

The sparse tool complains as follows:

drivers/platform/x86/thinkpad_acpi.c:9636:25: warning:
 symbol 'dev_attr_charge_start_threshold' was not declared. Should it be static?
drivers/platform/x86/thinkpad_acpi.c:9642:25: warning:
 symbol 'dev_attr_charge_stop_threshold' was not declared. Should it be static?

Those variables are not used outside of thinkpad_acpi.c, so this
commit marks them static.

Fixes: e33929537b ("platform/x86: thinkpad_acpi: use standard charge control attribute names")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/thinkpad_acpi.c

@@ -9633,13 +9633,13 @@ static ssize_t charge_control_end_threshold_store(struct device *dev,
 
 static DEVICE_ATTR_RW(charge_control_start_threshold);
 static DEVICE_ATTR_RW(charge_control_end_threshold);
-struct device_attribute dev_attr_charge_start_threshold = __ATTR(
+static struct device_attribute dev_attr_charge_start_threshold = __ATTR(
 	charge_start_threshold,
 	0644,
 	charge_control_start_threshold_show,
 	charge_control_start_threshold_store
 );
-struct device_attribute dev_attr_charge_stop_threshold = __ATTR(
+static struct device_attribute dev_attr_charge_stop_threshold = __ATTR(
 	charge_stop_threshold,
 	0644,
 	charge_control_end_threshold_show,