libxml2: revert commit breaking patchs in cmake file

libxml2: revert commit breaking patchs in cmake file

Make a revert of commit which breaks cross-compilation of depending
components.
This commit changes path calculation from relative to cmake file to
absolute from includedir, which points then the host /usr/include.
Submitted upstream ticket [1] to clarify how this should be fixed in
libxml2 upstream.

[1] https://gitlab.gnome.org/GNOME/libxml2/-/issues/898#note_2452864

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>