closingd: ignore announcement_signatures during closing.

This could always happen, but now we're more correct about send it, it's
a possible race that I've seen in CI.

We can never get a PONG (that's now handled by connectd directly), but we
can definitely see this:

```
2025-05-15T02:29:14.7403081Z         # Even with onchain anchor channel, it still keeps reserve (just in case!).
2025-05-15T02:29:14.7403766Z >       l1.rpc.close(l2.info['id'])
2025-05-15T02:29:14.7404069Z 
2025-05-15T02:29:14.7404248Z tests/test_opening.py:2536:
...
2025-05-15T02:29:14.7413090Z >       return self.sock.recv(length)
2025-05-15T02:29:14.7413353Z E       Failed: Timeout >1800.0s
...
2025-05-15T02:29:14.8097229Z lightningd-1 2025-05-15T01:50:59.538Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-closingd-chan#1: billboard perm: Expected closing_signed: 0103fad81b49d5e367cd785e3088b9acf356a7984a17a1ccda86d09da41438840b08000067000001000078c3314666731e339c0b8434f7824797a084ed7ca3655991a672da068e2c44cb53b57b53a296c133bc879109a8931dc31e6913a4bda3d58559b99b95663e6d52679cabaccc6e39dbd95a4dac90e75a258893c3aa3f733d1b8890174d5ddea8003cadffe557773c54d2c07ca1d535c4bf85885f879ae466c16a516e8ffcfec174
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

closingd/closingd.c

@@ -267,8 +267,8 @@ receive_offer(struct per_peer_state *pps,
 		 */
 		else if (fromwire_peektype(msg) == WIRE_SHUTDOWN)
 			msg = tal_free(msg);
-		/* channeld may have sent ping: ignore pong! */
-		else if (fromwire_peektype(msg) == WIRE_PONG)
+		/* We can get announcement signatures: too late! */
+		else if (fromwire_peektype(msg) == WIRE_ANNOUNCEMENT_SIGNATURES)
 			msg = tal_free(msg);
 	} while (!msg);