platform/surface: aggregator_registry: Make symbol 'ssam_base_hub_group' static

platform/surface: aggregator_registry: Make symbol 'ssam_base_hub_group' static

The sparse tool complains as follows:

drivers/platform/surface/surface_aggregator_registry.c:355:30: warning:
 symbol 'ssam_base_hub_group' was not declared. Should it be static?

This symbol is not used outside of surface_aggregator_registry.c, so this
commit marks it static.

Fixes: 797e78564634 ("platform/surface: aggregator_registry: Add base device hub")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210309131500.1885772-1-weiyongjun1@huawei.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>