Bug#1146800: timg: ftbfs with openslide 4.0.1 in experimental

[Benjamin Gilbert]

Thanks for the context.  If I'm understanding #826048 correctly, that
bug describes non-fatal warnings about optional dependencies.  Here,
the additional -dev packages are necessary for libopenslide-dev to be
useful at all, so it seems to me that libopenslide-dev should add
dependencies on them.  I've filed #1146883 to discuss further.