Make generating dependent include files for C++ the default.

--gen-includes is now deprecate but still accepted.
--no-includes can be used instead when needed.

Change-Id: I2cd46d193032b9b7c31c76c6f655e9333d3a393a
Tested: on Linux.

tests/test.cpp

@@ -291,6 +291,7 @@ void ParseProtoTest() {
 
   // Generate fbs.
   flatbuffers::GeneratorOptions opts;
+  opts.include_dependence_headers = false;
   auto fbs = flatbuffers::GenerateFBS(parser, "test", opts);
 
   // Ensure generated file is parsable.