netlink: specs: support generating code for genl socket priv

The family struct is auto-generated for new families, support
use of the sock_priv_* mechanism added in commit a7311324


("genetlink: introduce per-sock family private storage").

For example if the family wants to use struct sk_buff as its
private struct (unrealistic but just for illustration), it would
add to its spec:

  kernel-family:
    headers: [ "linux/skbuff.h" ]
    sock-priv: struct sk_buff

ynl-gen-c will declare the appropriate priv size and hook
in function prototypes to be implemented by the family.

Link: https://lore.kernel.org/r/20240308190319.2523704-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>