package/systemd: add missing path options

If paths are not set, then meson will search the host system for the
binaries (or the target, where those binaries are not yet installed).
So add the missing paths.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Reviewed-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>