add version.mli

Summary:
public
version.ml changes at each commit, which triggers the recompilation of all .ml
files that depend on it. Adding version.mli should prevent that.

Reviewed By: jberdine

Differential Revision: D2748986

fb-gh-sync-id: 5a59a14

infer/src/backend/version.mli

@@ -0,0 +1,11 @@
+(*
+ * Copyright (c) 2015 - present Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *)
+
+val versionString : string
+val versionJson : string