bankd: send IPA CCM ID_ACK after receiving ID_ACK

It is customary in the IPA protocol that a a server side
responds with an ID_ACK if the client sends an ID_ACK.  Due
to the lack of any protocol specification, it's unclear why
exactly, but we know it does happen.

osmo-remsim-bankd so far failed to implement this, which is
not directly a problem as the only user (osmo-remsim-client)
didn't care.  However, when executing TTCN3 test cases,
the IPA_Emulation expects that ID_ACK and related test fail.

Change-Id: Ie55c9d5c435df786e97ec3900837bb21ab80140a

src/bankd/bankd_main.c

@@ -747,6 +747,8 @@ restart_wait:
 		switch (hh->data[0]) {
 		case IPAC_MSGT_PING:
 			return ipa_ccm_send_pong(worker->client.fd);
+		case IPAC_MSGT_ID_ACK:
+			return ipa_ccm_send_id_ack(g_worker->client.fd);
 		default:
 			LOGW(worker, "IPA CCM 0x%02x not implemented yet\n", hh->data[0]);
 			break;