[xml] xml and STL with MSVC

[Timothee Besset <timo qeradiant com>]

I came across a small issue that occurs when using both STL and XML. If there's an #include <list> statement 
before #include <libxml/parser.h> and STL is configured not to use the std:: namespace, then the declaration 
of the following functions is problematic:

- xmlParseBalancedChunkMemory
- xmlParseExternalEntity
- xmlParseCtxtExternalEntity

because they use "list" as variable identifier. it seems that gcc is not affected, it can tell the difference 
between a variable name and a type .. but MSVC bails out.

TTimo