Add missing license field in opam file

9fe59d2 added the license field to the
dune-project file, but the opam file wasn't regenerated. This commit
fixes the error, but, it would be nice if ocaml-ci caught this for us.

opam-ci-check.opam

@@ -12,6 +12,7 @@ authors: [
   "Puneeth Chaganti <[email protected]>"
   "Shon Feder <[email protected]>"
 ]
+license: "Apache-2.0"
 tags: ["opam" "ci" "lint" "reverse dependency"]
 homepage: "https://github.com/ocurrent/opam-ci-check"
 doc: "https://www.ocurrent.org/opam-ci-check/opam-ci-check/index.html"