mailbox: mtk-gpueb: Add missing 'static' to mailbox ops struct

mailbox: mtk-gpueb: Add missing 'static' to mailbox ops struct

mtk_gpueb_mbox_ops should be declared static. However, due to its const
nature, this specifier was missed, as it compiled fine without it and
with no warning by the compiler.

arc-linux-gcc (GCC) 12.5.0 doesn't seem to like it however, so add the
static to fix that.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510100629.3nGvrhEU-lkp@intel.com/
Fixes: dbca0eabb821 ("mailbox: add MediaTek GPUEB IPI mailbox")
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>