Replace #include "tinyxml2/tinyxml2.h" to #include "tinyxml2.h" in new cocostudio code. This should fix build with cmake when USE_PREBUILT_LIBS=NO. Because FindTinyXML2.cmake find tinyxml2.h header (without prefix dir). And some systems install tinyxml2 includes without special directory.
Also this inclusion style match other existing files such as cocos/platform/CCFileUtils.cpp
Hide tinyxml2 from user visible header.
Fix couple of warnings. Make forward declarations to match to real declarations.
Merge pull request #9495 from vovkasm/pull/fix-tinyxml-usage

Fix usage of tinyxml2 in cocostudio code