Different versions of RWSTL have different headers; revert the previous change and instead introduce PUGIXML_NO_STL_FWDDECL macro (that disables forward declaration of STL classes, which is not compliant).

git-svn-id: http://pugixml.googlecode.com/svn/trunk@857 99668b35-9821-0410-8761-19e4c4f06640