staging: vchiq: Make vchiq_add_service() local

The function is being exported although there is no use for it outside
of vchiq's core code. Keep it local then.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-39-nsaenzjulienne@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>