Take $DESTDIR variable into account when installing sysstat service used by systemd.

Previous code for installing sysstat using systemd needed rw permissions
to the root directory. Yet some users and distributions want to install
files into $(DESTDIR) before actually installing into the root directory.
So take $DESTDIR variable into account.

Signed-off-by: Sebastien GODARD <sysstat@orange.fr.fake>