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>