windfarm: make symbol 'wf_thread' static

The sparse tool complains as follows:

drivers/macintosh/windfarm_core.c:59:20: warning:
 symbol 'wf_thread' was not declared. Should it be static?

This symbol is not used outside of windfarm_core.c, so this
commit marks it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210407125738.4138480-1-yukuai3@huawei.com

drivers/macintosh/windfarm_core.c

@@ -56,7 +56,7 @@ static BLOCKING_NOTIFIER_HEAD(wf_client_list);
 static int wf_client_count;
 static unsigned int wf_overtemp;
 static unsigned int wf_overtemp_counter;
-struct task_struct *wf_thread;
+static struct task_struct *wf_thread;
 
 static struct platform_device wf_platform_device = {
 	.name	= "windfarm",