At some point we may want to add xz-native to ASSUME_PROVIDED. This
allows that to work whilst still allowing access to liblzma for
those things which need it (e.g. pixz).
(From OE-Core rev: 5c69c561a76cb10d7896ae0a0399190f11b2e0ca)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>