vty: fix: restore removed DNAT category as deprecated

This logging category has been removed in [1], what caused test
case execution failures on our Jenkins (since build #930). The
problem is that configuration files may still contain this
category in 'logging' section (see [2], [3], [4]), and
osmo-bsc would refuse to start because of that.

[1] Id965295dfe04f8bd5ce831db70c86f67b8dc290b
[2] Ie2afacfc15589c26238214cddc00baaf80e993c1
[3] I266d6f6ed54d1457b1ca63b87fc1c29f6dd40caf
[4] If02272c08ba2df37d1295d09c104d11f96abbe1e

Change-Id: I111362d19aba325889bada5a46eea62343c30033
diff --git a/src/osmo-bsc/osmo_bsc_main.c b/src/osmo-bsc/osmo_bsc_main.c
index de84060..ad5a3a9 100644
--- a/src/osmo-bsc/osmo_bsc_main.c
+++ b/src/osmo-bsc/osmo_bsc_main.c
@@ -845,6 +845,7 @@
 	ctrl_vty_init(tall_bsc_ctx);
 	logging_vty_add_deprecated_subsys(tall_bsc_ctx, "cc");
 	logging_vty_add_deprecated_subsys(tall_bsc_ctx, "mgcp");
+	logging_vty_add_deprecated_subsys(tall_bsc_ctx, "nat");
 
 	/* Initialize SS7 */
 	OSMO_ASSERT(osmo_ss7_init() == 0);