netlink-protocol: Remove definition of struct sockaddr_nl.

This struct is used only in netlink-socket.c, which is only used on Linux,
which in turn gets the definition from <linux/netlink.h>.  On Windows the
definition actually causes a small amount of trouble because Windows does
not define sa_family_t (despite POSIX), so it's better to remove it.

Even if other platforms adopt Netlink, I have no reason to believe that
they will use the same sockaddr format as Linux.

CC: Saurabh Shah <ssaurabh@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>