workaround const qualifier discard

Summary:
In regular xcode this was warning and being ignores. Not the working is handled as an error.
This diff is only a workaround so we wont get those errors .
```
eden/scm/edenscm/mercurial/cext/osutil.c:745:49: error: passing 'const char *' to parameter of type 'char *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
  ret = _listdir_batch(path, pathlen, keepstat, skip, &fallback);
                                                ^~~~
eden/scm/edenscm/mercurial/cext/osutil.c:586:11: note: passing argument to parameter 'skip' here
    char* skip,
```

Reviewed By: mzlee

Differential Revision: D27136440

fbshipit-source-id: 00d61fd00e3ed8e23643ea69b5a82dbeb5e742ce

eden/scm/edenscm/mercurial/cext/osutil.c

@@ -580,10 +580,10 @@ int attrkind(attrbuf_entry* entry) {
 #define LISTDIR_BATCH_SIZE 50
 
 static PyObject* _listdir_batch(
-    char* path,
+    const char* path,
     int pathlen,
     int keepstat,
-    char* skip,
+    const char* skip,
     bool* fallback) {
   PyObject *list, *elem, *stat = NULL, *ret = NULL;
   int kind, err;