[merlin] disable warning 60 (unused module declaration)

Summary:
Every module declared but not used in the same source file is warned about
currently. Disable the noisy warning.

For instance, before this diff (and after "M-x merlin-restar-process"):

1. open AbstractInterpreter.ml
2. merlin shows a warning for `module Make` inside emacs

Reviewed By: jberdine

Differential Revision: D5621383

fbshipit-source-id: c175e5d

infer/src/.merlin

@@ -15,7 +15,7 @@ PKG xmlm
 PKG yojson
 PKG zip
 FLG -principal -safe-string -short-paths -strict-formats -strict-sequence
-FLG -w +a-4-9-40-41-42-45-48
+FLG -w +a-4-9-40-41-42-45-48-60
 FLG -open InferBaseStdlib -open InferGenerated -open InferModules
 S backend
 S base