meson: do not fail if rsync is not installed with meson 0.57.2

Fixes: https://github.com/mesonbuild/meson/issues/8641

Backport fix from,
https://github.com/systemd/systemd/commit/7c5fd25119a495009ea62f79e5daec34cc464628



Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>