Add libplacebo build system deps

We need access to python3-setuptools and python3-mako in order to successfully run the libplacebo meson.build.

The alternative would be to somehow install this as part of the contribs, but I think it makes more sense to add it directly to the image as it's for the build system itself. In particular, the setuptools requirement is actually inherent to meson (import('python') requires it).