flatpak: Fix the build, adding the missing patch
SuiteSparse was added as a dependency in 8ee11d8d, but with a missing patch. It's probably only in the developer clone and was left uncommitted.
Fortunately, a comment indicates that the module definition was copied from Octave, which is on Flathub:
This commit just adds the patch as it is in the Octave repository, fixing the Flatpak build of this project.