multi: add GetBlockHeader to BlockChainIO

contractcourt/channel_arbitrator_test.go

@@ -187,6 +187,10 @@ func (*mockChainIO) GetBlock(blockHash *chainhash.Hash) (*wire.MsgBlock, error)
 	return nil, nil
 }
 
+func (*mockChainIO) GetBlockHeader(*chainhash.Hash) (*wire.BlockHeader, error) {
+	return nil, nil
+}
+
 type chanArbTestCtx struct {
 	t *testing.T