libxml2: update to 2.12.6

libxml2: update to 2.12.6

Release Notes:
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.12.6

Changelog (taken from the release notes):
- Regressions
  - 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

Compile-tested: x86_x64, Q35 VM, OpenWRT SNAPSHOT r26135-a8bfdf2ed4d9
Run-tested: x86_x64, Q35 VM, OpenWRT SNAPSHOT r26135-a8bfdf2ed4d9
Signed-off-by: Pascal Ernster <git@hardfalcon.net>
Link: https://github.com/openwrt/openwrt/pull/18280
Signed-off-by: Robert Marko <robimarko@gmail.com>