[PATCH] Missing elpi_elpi in src/dune

[SnarkBoojum]

Trying to update the Coq ecosystem in Debian, I had a strange issue compiling mathcomp-multinomials: it didn't find elpi_elpi's dummy.

After much head scratching, I discovered that mathcomp-multinomials is one of the rare packages using both dune and declaring coq 0.8 in its dune-project (not 0.6) : adding "elpi_elpi" to the "theories" field in src/dune fixes the problem.

(Aside: looking at the github code, there's no use of dune ; but the tarball for the last release does - even if it's README.md still says to use make...)