main/alpine-base: add mdev-conf to alpine-base dependancy list

With the various restructuring changes in Alpine Edge around mdev/mdevd/udev I have found during testing on a system being converted to eudev/udev use that whenever "setup-devd udev" does a "apk add eudev udev-init-scripts udev-init-scripts-openrc" this not only automatically triggers the removal of the busybox-mdev-openrc package but also the removal of mdev-conf.

mdev-conf contains various mdev configuration related files that are required on all Alpine systems regardless of which of mdev/mdevd/udev is in use - if mdev-conf is removed then any time mkinitfs rebuilds the initramfs file (such as when triggered by a kernel package update) the resultant initramfs will have NO mdev support and so the machine will fail to boot.

This change ensures that mdev-conf is always a dependancy.