[arch-general] Kpartx should be in the repos and archiso for enabling encrypted GPT install

[Neven Sajko]

> This is interesting, partprobe seems to be an even lighter alternative.

It seems that partprobe indeed also works for device mapper maps.
Relevant links:

* A commit to the (c)fdisk man page that suggests using kpartx or
partprobe when the BLKRRPART ioctl fails (I wonder why that was not
upstreamed):

https://git.devuan.org/CenturionDan/util-linux/commit/c64061c9b306f2db49ba97d066b8487f66a914fa?w=1

* Some relevant Archlinux BBS threads, one commenter there even
recommends kpartx over partprobe for some reason:

https://bbs.archlinux.org/viewtopic.php?id=189537
https://bbs.archlinux.org/viewtopic.php?id=190672