[FAB-1651] Fix broadcast_timestamp client

https://jira.hyperledger.org/browse/FAB-1651

The broadcast_timestamp client has gotten slightly out of sync with the
orderer code, and does not work.  It is a simple fix to set the
signature header field as seen in the diff.

Change-Id: I70c00b356ac65b74d06e59f1bd05eab0e9f4097d
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

orderer/sample_clients/broadcast_timestamp/client.go

@@ -46,6 +46,7 @@ func (s *broadcastClient) broadcast(transaction []byte) error {
 			ChainHeader: &cb.ChainHeader{
 				ChainID: s.chainID,
 			},
+			SignatureHeader: &cb.SignatureHeader{},
 		},
 		Data: transaction,
 	})