Fix build on systems that lack NETLINK_SOCK_DIAG definition

NETLINK_INET_DIAG was introduced in linux v2.6.14,
NETLINK_SOCK_DIAG was added much later in linux v3.3.

* net.c [!NETLINK_SOCK_DIAG && NETLINK_INET_DIAG]
(NETLINK_SOCK_DIAG): Define.
* socketutils.c: Likewise.
* tests/netlink_unix_diag.c: Likewise.
3 files changed