libxml2: synchronize with mingw-w64-pathtools

Just like `mingw-w64-curl`, `mingw-w64-libxml2` is now also built with a
literal copy of `mingw-w64-pathtools`' source code.

This adds the `get_dll_path()` function (which is unused by the
`libxml2` code), and it also forward-ports the patch where we now
preserve UNC paths when normalizing/simplifying paths.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>