archiver-common: add mkinitcpio support to private-etc

mkinitcpio (used to generate initramfs images) supports several
compression formats:
https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio/-/blob/master/mkinitcpio.conf#L54-L64.

On Arch Linux (based distributions) at least this implies the supported
archivers to have access to mkinitcpio-related files under /etc.

This was no problem before 29da82d added `private-etc` to
`archivers-common.profile`.

This adds the now needed extra private-etc items to
archiver-common.profile, for mkinitcpio's supported compressors (which
seem to be at least cpio, gzip and zstd).

Relates to #5610.

etc/profile-a-l/archiver-common.profile

@@ -44,7 +44,7 @@ x11 none
 
 private-cache
 private-dev
-private-etc
+private-etc mkinitcpio*
 
 dbus-user none
 dbus-system none