libxml2: update to 2.13.4

libxml2: update to 2.13.4

Release Notes:
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.0
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.1
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.2
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.3
https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.4

Small size reduction:
415095 bin/packages/mips_24kc-old/base/libxml2_2.12.6-r1_mips_24kc.ipk
 87175 bin/packages/mips_24kc-old/base/libxml2-dev_2.12.6-r1_mips_24kc.ipk
 20190 bin/packages/mips_24kc-old/base/libxml2-utils_2.12.6-r1_mips_24kc.ipk
398070 bin/packages/mips_24kc-new/base/libxml2_2.13.4-r1_mips_24kc.ipk
 86760 bin/packages/mips_24kc-new/base/libxml2-dev_2.13.4-r1_mips_24kc.ipk
 19479 bin/packages/mips_24kc-new/base/libxml2-utils_2.13.4-r1_mips_24kc.ipk

Link: https://github.com/openwrt/openwrt/pull/16593
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>