[opam] add zmq as optional dependency

opam/opam

 opam-version: "2.0"
 name: "frama-c"
 synopsis: "Platform dedicated to the analysis of source code written in C"
-version: "19.1"
+version: "20.0"
 maintainer: "francois.bobot@cea.fr"
 authors: [
   "Michele Alberti"
@@ -110,6 +110,7 @@ depopts: [
   "coq"
   "mlgmpidl"
   "apron"
+  "zmq"
 ]
 
 conflicts: [