Clean up another c qualifier error

Summary:
Filenames are `const char *`, not `char *` and the `statfs` interface
respects this; we should as well.

Reviewed By: grievejia

Differential Revision: D28464007

fbshipit-source-id: 10cbe9875d9f036a075a7b0ca09834170555a431

Author: stroxler

source/hack_parallel/hack_parallel/utils/files.c

@@ -37,7 +37,7 @@ value hh_is_nfs(value filename_v) {
   CAMLparam1(filename_v);
 #ifdef __linux__
   struct statfs buf;
-  char* filename = String_val(filename_v);
+  const char* filename = String_val(filename_v);
   int success = statfs(filename, &buf);
   if (success != 0) {
     caml_failwith("statfs failed");