Increase side of mapping response field.

I've increased the size of the response field in a DUNDi mapping because of
some documentation I'm writing. Previously it was set to AST_MAX_EXTENSION which
is only 80 characters, which is far too small when you're using some dialplan
functions to craft a response. The example I'm using is:

extensions =>
RegisteredDevices,0,SIP,dundi:very_awesome_password/${IF($[${DB_EXISTS(phones/${NUMBER}/device)}]?${DB(phones/${NUMBER}/device)}:None)},nopartial



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@300082 65c4cc65-6c06-0410-ace0-fbb531ad65f3

pbx/pbx_dundi.c

@@ -286,7 +286,7 @@ struct dundi_mapping {
 	int options;
 	int tech;
 	int dead;
-	char dest[AST_MAX_EXTENSION];
+	char dest[512];
 	AST_LIST_ENTRY(dundi_mapping) list;
 };