oc2gbts_mgr: use msgb_talloc_ctx_init() instead of msgb_set_talloc_ctx()

The function msgb_set_talloc_ctx() is deprecated. Lets use
msgb_talloc_ctx_init() as suggested.

Change-Id: I61f80a7b11e9117e8b71c49b7d81d2f959f3150d

src/osmo-bts-oc2g/misc/oc2gbts_mgr.c

@@ -274,7 +274,7 @@ int main(int argc, char **argv)
 
 	tall_mgr_ctx = talloc_named_const(NULL, 1, "bts manager");
 	tall_msgb_ctx = talloc_named_const(tall_mgr_ctx, 1, "msgb");
-	msgb_set_talloc_ctx(tall_msgb_ctx);
+	msgb_talloc_ctx_init(tall_msgb_ctx, 0);
 
 	mgr_log_init();