upgpkg: 12.1.0-6: add `qt5-imageformats` as optdep
After a discussion in the project's issue tracker: https://github.com/flameshot-org/flameshot/issues/2989
I think that adding "qt5-imageformats" as an optional dependency for flameshot
is a good idea, since it adds additional export formats to it.
It's already an optional dependency for the gwenview
image viewer which has the same idea, so I think there is no problem.