libsystemd: export sd-netlink

It was added originally in 65f568bbeb. The API is
has stabilized pretty much, and generally follows the usual style for
libsystemd.  We've held it as a public-but-private library for almost 10 years,
let's export it.

sd_netlink_sendv() and sd_nfnl_nft_*() are excluded.

libsystemd.so seems to grow by 12k.

src/shared/firewall-util-nft.c

@@ -18,6 +18,8 @@
 #include "firewall-util-private.h"
 #include "in-addr-util.h"
 #include "macro.h"
+#include "netlink-internal.h"
+#include "netlink-util.h"
 #include "socket-util.h"
 #include "time-util.h"