Add AUR package 'normalize' to optdepends.

normalize is required for the 'Normalize volume levels' feature, which can be seen when burning audio CDs. normalize was once listed in depends before a commit in 2009, but then it has not been listed even in optdepends since.

A fact of note is that the normalize package WAS in the official repositories, but apparently in 2018 it was demoted to AUR. However, to this date, the k3b app still optionally depends on this app for the aforementioned feature.

I have personally consulted with community members with the 'AUR package as an official package optdepend' issue and decided that there isn't sufficient official statement to deter such changes to be proposed for an official package. Hence, this merge request.