Update external dependency: tinyxml2. (#256)

Modifications based on tinyxml2 ver 6.1.0 (https://github.com/leethomason/tinyxml2/tree/f26a547fdc216bca49ea2b9b61ed89bf2f301eb3)
* Import-export preprocessor (USE_IMPORT_EXPORT)
* Aws memory management apis (Aws::NewArray, Aws::New, Aws::Delete)
* Wrap everything in Aws namespace (namespace Aws { namespace External { namespace tinyxml2 {...}}})
* Include with <> rather than "" (#include <aws/core/external/tinyxml2/tinyxml2.h>)

Author: wps132230