macintosh/windfarm_pid: Add header file macro definition

I think the header file could avoid redefinition errors at compile time
by adding macro definitions.
Signed-off-by: Li zeming <zeming@nfschina.com>
[mpe: Add endif comment]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220707015949.3733-1-zeming@nfschina.com

drivers/macintosh/windfarm_pid.h

 /* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _WINDFARM_PID_H
+#define _WINDFARM_PID_H
+
 /*
  * Windfarm PowerMac thermal control. Generic PID helpers
  *
@@ -82,3 +85,5 @@ struct wf_cpu_pid_state {
 extern void wf_cpu_pid_init(struct wf_cpu_pid_state *st,
 			    struct wf_cpu_pid_param *param);
 extern s32 wf_cpu_pid_run(struct wf_cpu_pid_state *st, s32 power, s32 temp);
+
+#endif /* _WINDFARM_PID_H */