Add missing upper bound for mtl package

agda2hs.cabal

@@ -52,7 +52,8 @@ executable agda2hs
                        bytestring           >= 0.11.5  && < 0.13,
                        containers           >= 0.6     && < 0.8,
                        unordered-containers >= 0.2.19  && < 0.3,
-                       mtl                  >= 2.2     && < 2.3 || >= 2.3.1,
+                       mtl                 (>= 2.2     && < 2.3)
+                                        || (>= 2.3.1   && < 2.4),
                        transformers         >= 0.6     && < 0.7,
                        monad-control        >= 1.0     && < 1.1,
                        directory            >= 1.2.6.2 && < 1.4,