macintosh/windfarm: Make symbol 'pm121_sys_state' static

The sparse tool complains as follows:

drivers/macintosh/windfarm_pm121.c:436:24: warning:
 symbol 'pm121_sys_state' was not declared. Should it be static?

This symbol is not used outside of windfarm_pm121.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/20210407125712.4138033-1-yukuai3@huawei.com

drivers/macintosh/windfarm_pm121.c

@@ -433,7 +433,7 @@ struct pm121_sys_state {
 	struct wf_pid_state	pid;
 };
 
-struct pm121_sys_state *pm121_sys_state[N_LOOPS] = {};
+static struct pm121_sys_state *pm121_sys_state[N_LOOPS] = {};
 
 /*
  * ****** CPU Fans Control Loop ******