platform/x86: intel_scu_ipcutil: Make scu static

The variable is only used internally and has no external users,
so it should me made static.

Compile-tested only.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20240223163901.13504-3-W_Armin@gmx.de


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>