Fix FTBFS with Ruby 3.0
This is a patch to fix #996313
- d/patches/996313-fix-ext-build-with-ruby3.patch: Add patch.
- Use 'include <ruby/st.h>' to fix build with Ruby 3 (closes: #996313).
- d/patches/series: Enable new patch.
The NMU has been uploaded to the DELAYED/2 queue.