package/systemd: also fix rpath for nspawn

Fixes:
    http://autobuild.buildroot.org/results/e03ae6a3209eea00459b94cee9c10fd4f2184fec/



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Jérémy Rosen <jeremy.rosen@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>