libxml2: Update to version 2.15.0

libxml2: Update to version 2.15.0

- Update from version 2.14.6 to 2.15.0
- Update of rootfile
- Changelog
    2.15.0
      Major changes
	The API documentation is now generated with Doxygen. Building the
	 documentation requires the new --with-docs configuration option as well
	 as Doxygen, xsltproc and the DocBook 4 XSLT stylesheets.
	The Python bindings are disabled by default now. Building the bindings
	 also requires Doxygen.
	Support for Schematron is now disabled by default.
	The parser option XML_PARSE_UNZIP is now required to read compressed data.
	HTML serialization and handling of character encodings is more in line
	 with the HTML5 spec now.
	More accessors for xmlParserCtxt were added.
      Deprecations
	More internal functions and struct members were deprecated.
      Removals
	The built-in HTTP client and support for LZMA compression were removed.
	The custom Windows build system in `win32` was removed in favor of CMake.
      Planned removals
	The Python bindings and support for Schematron validation are planned to
	 be removed in the 2.16 release.
	The following features are considered for removal:
	- Modules API (xmlmodule.h)
	- Support for zlib compressed file I/O
	RELAX NG support is still in a bad state and a long-term removal
	 candidate.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>