Explicitly ask for python-yaml

Right now it gets pulled in automatically as a dependency of something
we also pre-install, but we should mark it down here so it never goes
away by accident as a result of other package changes, because then
kde-builder would stop working.

mkosi.conf.d/70-packages-build.conf

@@ -10,4 +10,5 @@ Packages=boost
          arch-install-scripts
          pacman
          python-setproctitle # For kde-builder
+         python-yaml # For kde-builder