xz: Revert back to version 5.4.5 due to backdoor issue

xz: Revert back to version 5.4.5 due to backdoor issue

- xz version 5.6.0 and 5.6.1 discovered to have been backdoored by what looks to have
   been one of the xz devs.
- IPFire looks not to be affected by the problem as we don't patch openssh to be linked
   with liblzma
- However due to question marks about what else might be in these 5.6.x versions it is
   better to revert back to a version that did not have the build-to-host.m4 file with the
   code that modifies the build if it meets certain criteria.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>