tools: ynl: Fix genlmsg header encoding formats

The pack strings use 'b' signed char for cmd and version but struct
genlmsghdr defines them as unsigned char. Use 'B' instead.

Fixes: 4e4480e8

 ("tools: ynl: move the cli and netlink code around")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230319193803.97453-1-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>