Add synopsis so make build-dep does not complain

Nitpick, yes, but I've seen the complaint enough times today to bother.

That complaint is only useful to maintainers, and it's even in color.

```
[WARNING] Failed checks on coq-iris-builddep package definition from source at file:///Users/pgiarrusso/git/Coq/0IRIS-SWITCH/iris/build-dep:
    error 57: Synopsis and description must not be both empty
```

opam

 opam-version: "1.2"
 name: "coq-iris"
+synopsis: "This is the Coq development of the Iris Project"
 maintainer: "Ralf Jung <jung@mpi-sws.org>"
 authors: "The Iris Team"
 homepage: "http://iris-project.org/"