libxml2: upgrade 2.12.5 -> 2.12.6

libxml2: upgrade 2.12.5 -> 2.12.6

Changelog:
===========
- parser: Fix detection of duplicate attributes in XML namespace
- xmlreader: Fix xmlTextReaderConstEncoding
- html: Fix htmlCreatePushParserCtxt with encoding
- xmllint: Return error code if XPath returns empty nodeset

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0bc673b0de08e02ff01ec9ad3daf0bb41662da40)
Signed-off-by: Steve Sakoman <steve@sakoman.com>