[infer] Fix `make deadcode`

Summary:
```
[*ERROR**][99281] Entering directory '/Users/scho/infer/infer/src'
[*ERROR**][99281] File "istd/IResult.mli", line 8, characters 25-43:
[*ERROR**][99281] Error: Unbound module Result.Monad_infix
[*ERROR**][99281] make[1]: *** [detect_dead_code] Error 1
make: *** [real_deadcode] Error 2
```

because of no `open! IStd`.

Reviewed By: dulmarod

Differential Revision: D19948195

fbshipit-source-id: 4f36bcb47

infer/src/istd/IResult.ml

@@ -4,6 +4,9 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *)
+
+open! IStd
+
 module Let_syntax = struct
   include Result.Monad_infix
 

infer/src/istd/IResult.mli

@@ -4,6 +4,9 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *)
+
+open! IStd
+
 module Let_syntax : sig
   include module type of Result.Monad_infix