mkosi: Don't skip initrd dependency when building a directory image

mkosi: Don't skip initrd dependency when building a directory image

mkosi now supports booting directory images in qemu using virtiofs.
However, until distribution kernels build the virtiofs driver directly
into the kernel, we need an initrd to make this work, so make sure to
pull in the initrd preset when building a directory image that could be
bootable to make this work.