message: Update EnableValidationWithCallback.cpp
  Quick patch that fixes the `EnableValidationWithCallback` sample:
  There's no `str()` method here, should use `c_str()`;
  *but* also, you can print the std::string directly to `std::cout`.
  Patches remove `.str()` from printing line when WIN32 is not defined.
diff: @@ -82,7 +82,7 @@ VkBool32 debugMessageFunc(VkDebugUtilsMessageSeverityFlagBitsEXT messageSeverity
  #ifdef _WIN32
  MessageBox(NULL, message.c_str(), "Alert", MB_OK);
  #else
  - std::cout << message.str() << std::endl;
  + std::cout << message << std::endl;
  #endif
  return false;
  