Disable libxml2 on Windows for now.

Newer versions of CMake include a "smarter" FindLibxml2 package.
In theory this is a good thing, but on Windows it's now smart
enough to find the version that comes with Gnuwin32, which doesn't
appear to be a valid libxml2 distribution.  Or at the very least,
LLDB currently uses some header files from libxml2 that are not
part of this distribution.

Nobody on Windows is using any of this functionality right now
anyway, so just disable it.

llvm-svn: 250709