alsa-utils: add optional systemd dependency

Fixes (reported by Yann E. MORIN, [1]):

/usr/bin/install -D -m 0644 .../build/alsa-utils-1.1.6/alsactl/alsa-restore.service .../target/usr/lib/systemd/system/alsa-restore.service
/usr/bin/install: cannot stat '.../build/alsa-utils-1.1.6/alsactl/alsa-restore.service': No such file or directory

[1] http://lists.busybox.net/pipermail/buildroot/2018-November/236355.html



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>