fs/common: build host-xz when xz compression is used

When a filesystem image is xz-compressed, the XZ variable is used to refer
to the xz compression tool, but it is not necessarily available. Add a
proper dependency to fix that.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>