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

include/linux/genl_magic_func.h

@@ -209,7 +209,7 @@ static int s_name ## _from_attrs_for_change(struct s_name *s,		\
  * Magic: define op number to op name mapping				{{{1
  *									{{{2
  */
-const char *CONCAT_(GENL_MAGIC_FAMILY, _genl_cmd_to_str)(__u8 cmd)
+static const char *CONCAT_(GENL_MAGIC_FAMILY, _genl_cmd_to_str)(__u8 cmd)
 {
 	switch (cmd) {
 #undef GENL_op