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 <[e-mail]>
[mpe: Add endif comment]
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

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
  *
···
 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 */