chan_vpb.cc: Fix compile errors.

Fix the usual compile problem when someone adds a new callback to struct
ast_channel_tech.

Change-Id: I9bdeb8a8cc65f03b2d6e4f2eb5809af47c906c32

channels/chan_vpb.cc

@@ -372,6 +372,7 @@ static struct ast_channel_tech vpb_tech = {
 	.call = vpb_call,
 	.hangup = vpb_hangup,
 	.answer = vpb_answer,
+	.answer_with_stream_topology = NULL,
 	.read = vpb_read,
 	.read_stream = NULL,
 	.write = vpb_write,
@@ -406,6 +407,7 @@ static struct ast_channel_tech vpb_tech_indicate = {
 	.call = vpb_call,
 	.hangup = vpb_hangup,
 	.answer = vpb_answer,
+	.answer_with_stream_topology = NULL,
 	.read = vpb_read,
 	.read_stream = NULL,
 	.write = vpb_write,