package/dbus: disable systemd for host build

package/dbus: disable systemd for host build

This fixes an issue if host-dbus happens to be rebuilt after systemd, in
which case it autodetects systemd support but then ignored the prefix
when installing unit files.  That means that is tries to write to the
host system's /usr/lib/ which fails.

There is no reason to build and install systemd support in the host
build, so disable it explicitly.

Signed-off-by: John Keeping <john@metanate.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>