firmware: arm_scmi: Add include guard to linux/scmi_protocol.h

If this header is include twice, it will generate loads of compile
time error with the following below error pattern. It was reported by
0day kbuild robot on a branch pushed with double inclusion by accident.

 	error: conflicting types for ‘...’
 	note: previous declaration of ‘...’ was here
	error: redefinition of ‘...’

Add a header include guard just in case.

Link: https://lore.kernel.org/r/20200403171018.1230-1-sudeep.holla@arm.com
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

include/linux/scmi_protocol.h

@@ -4,6 +4,10 @@
  *
  * Copyright (C) 2018 ARM Ltd.
  */
+
+#ifndef _LINUX_SCMI_PROTOCOL_H
+#define _LINUX_SCMI_PROTOCOL_H
+
 #include <linux/device.h>
 #include <linux/types.h>
 
@@ -319,3 +323,5 @@ static inline void scmi_driver_unregister(struct scmi_driver *driver) {}
 typedef int (*scmi_prot_init_fn_t)(struct scmi_handle *);
 int scmi_protocol_register(int protocol_id, scmi_prot_init_fn_t fn);
 void scmi_protocol_unregister(int protocol_id);
+
+#endif /* _LINUX_SCMI_PROTOCOL_H */