install first lib32-systemd as follow
$ sudo pacman -S lib32-systemd --assume-installed systemd
then
$ yaourt -Syua
you can also comment the line IgnorePkg=systemd libsystemd in /etc/pacman.conf, but if you do that pacman is able to install systemd and libsystemd as dependancies.
for me the first solution is better and prevent systemd conflicts but it's up to you :)
EDIT : i moved your topic in system administration because it isn't a bug, but a special stuff on your system