libxml2: don't grow dependency on liblzma if that happens to be installed

Change-Id: Ifd46fc5dc531bd5bdb489c7550476f9c07a0a976
(cherry picked from commit b202af672cf7d1825b68adc352374031bf68c6f2)
Reviewed-on: https://gerrit.libreoffice.org/7345
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>