Mark libev as unavailable on Windows

Author: punchagan

packages/conf-libev/conf-libev.4-11/opam

@@ -27,6 +27,7 @@ Libev is modelled (very loosely) after libevent and the Event perl
 module, but is faster, scales better and is more correct, and also more
 featureful. And also smaller. Yay."""
 bug-reports: "https://github.com/ocaml/opam-repository/issues"
+available: os != "win32"
 flags: conf
 extra-source "discover.ml" {
   src: