Fixed <libxml/xmlreader.h> not found compilation error on Xcode 5 with OS X Mavericks

Wasn't able to compile on OSX Mavericks due to XML library not found error. 
I'm not sure if this is a Mavericks specific problem, and isn't public release yet so this pull request might be for future reference.

I do not have previous OS so need to verify if it still builds.

http://stackoverflow.com/questions/6592193/how-to-solve-libxml-xmlreader-h-not-found-error-in-twitter-integration-in-ipho