libxml2: Add abi version

libxml2: Add abi version

The version of libxml2 was bumped from 2.13.6 to 2.14.5. Since version
2.14, libxml2 is not binary compatible with older versions. Therefore
add an abi version.

From the NEWS file:
Binary compatibility is restricted to versions 2.14 or newer. On ELF
systems, the soname was bumped from libxml2.so.2 to libxml2.so.16.

Signed-off-by: Jan Kardell <jan.kardell@telliq.com>
Link: https://github.com/openwrt/openwrt/pull/19983
(cherry picked from commit 420be05d90b2eccaa828cb126f87742f30b804b2)
Link: https://github.com/openwrt/openwrt/pull/19985
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>