Fix generated_by_local arg to build commmitment during splicing

`build_commitment_transaction`'s fifth argument is supposed to be
whether we're the one generating the commitment (i.e. because we're
signing rather than validating the commitment). During splicing,
this doesn't matter because there should be no async HTLC
addition/removal happening so the commitment generated wil be the
same in either case, but its still good to pass the correct bool.

Backport of 0f4e6c2

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -6252,7 +6252,7 @@ where
 			commitment_number,
 			&commitment_point,
 			false,
-			false,
+			true,
 			logger,
 		);
 		let counterparty_initial_commitment_tx = commitment_data.tx;