Avoid making AstData depend on libxml2 to compile.

We have some functions inside the AstData API to get the tree
in XML form, but it is not required at the moment to compile 
asterisk and we can disable that part of the API if we don't have
libxml2 support.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@260521 65c4cc65-6c06-0410-ace0-fbb531ad65f3