[C++]: added includes for messageHader. aeron-io#435

Author: tmontgomery

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -56,7 +56,8 @@ public void generateMessageHeaderStub() throws IOException
  try (Writer out = outputManager.createOutput(messageHeader))
  {
  final List<Token> tokens = ir.headerStructure().tokens();
- out.append(generateFileHeader(ir.namespaces(), messageHeader, null));
+ out.append(generateFileHeader(ir.namespaces(), messageHeader,
+ generateTypesToIncludes(tokens.subList(1, tokens.size() - 1))));
  out.append(generateClassDeclaration(messageHeader));
  out.append(generateFixedFlyweightCode(messageHeader, tokens.get(0).encodedLength()));
  out.append(generateCompositePropertyElements(