Makefile.am: run distcheck _with_ systemunitdir
Makefile.am: run distcheck _with_ systemunitdir

https://www.freedesktop.org/software/systemd/man/daemon.html
recommends to pass the unit dir as hint -that it should be treated
relative to the install base path- when running 'make distcheck'.

Fixes: 181816d ("Makefile.am: run distcheck without systemdunitdir")
Signed-off-by: Michael Heimpold <59934addbc17c0fa64a0bf4bb38537fc23439974@heimpold.de>

jluebbe/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc