Fix silent rebase conflict that broke tests

429cbe1 merged a PR that renamed
Offer::signing_pubkey to Offer::issuer_signing_pubkey. However, there was a
silent rebase conflict and a test added as part of
1059f5f did not get the memo and used the old
method name, breaking the test build.

Author: valentinewallace

lightning/src/ln/offers_tests.rs

@@ -2287,7 +2287,7 @@ fn no_double_pay_with_stale_channelmanager() {
 		.clear_paths()
 		.amount_msats(amt_msat)
 		.build().unwrap();
-	assert_eq!(offer.signing_pubkey(), Some(bob_id));
+	assert_eq!(offer.issuer_signing_pubkey(), Some(bob_id));
 	assert!(offer.paths().is_empty());
 
 	let payment_id = PaymentId([1; 32]);