libxml2: move to gitlab.gnome.org

libxml2: move to gitlab.gnome.org

The project has migrated from www.xmlsoft.org to gitlab.gnome.org.
Update the homepage accordingly, and use gnomebase to construct the
download URL, rather than including it in SRC_URI explicitly.

Note that the download is now in .xz format rather than .gz, so the
sha256sum is updated accordingly. Post-decompression tarballs are
identical, so there is no change to the libxml2 code.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8bc17ceb997f8f31a03e5f5efc41c03ef1df3add)
Signed-off-by: Steve Sakoman <steve@sakoman.com>