systemd: add zstd PACKAGECONFIG

systemd supports using zstd compression for journald and core files.
Add the necessary PACKAGECONFIG to enable zstd.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>