firmware: arm_scpi: Add include guard to linux/scpi_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.
This is based on the similar change in linux/scmi_protocol.h

        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/20200403171018.1230-2-sudeep.holla@arm.com
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

include/linux/scpi_protocol.h

@@ -4,6 +4,10 @@
  *
  * Copyright (C) 2014 ARM Ltd.
  */
+
+#ifndef _LINUX_SCPI_PROTOCOL_H
+#define _LINUX_SCPI_PROTOCOL_H
+
 #include <linux/types.h>
 
 struct scpi_opp {
@@ -71,3 +75,5 @@ struct scpi_ops *get_scpi_ops(void);
 #else
 static inline struct scpi_ops *get_scpi_ops(void) { return NULL; }
 #endif
+
+#endif /* _LINUX_SCPI_PROTOCOL_H */