genetlink: make _genl_cmd_to_str static

Primarily to silence warnings like:
warning: no previous prototype for 'xxx_genl_cmd_to_str' [-Wmissing-prototypes]

Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20230330102744.2128122-2-christoph.boehmwalder@linbit.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>