Fix type of getrlimit arg

[dalance]

At libc 0.2.56, the type of resource changed from c_int to __rlimit_resource_t on linux environment.
Therefore build failure occurs.

error[E0308]: mismatched types
  --> src/valgrind.rs:15:39
   |
15 |     let err = unsafe { libc::getrlimit64(resource, &mut rlim) };
   |                                          ^^^^^^^^ expected u32, found i32
help: you can convert an `i32` to `u32` and panic if the converted value wouldn't fit
   |
15 |     let err = unsafe { libc::getrlimit64(resource.try_into().unwrap(), &mut rlim) };
   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> src/valgrind.rs:33:23
   |
33 |     let rlim = getrlimit(libc::RLIMIT_NOFILE).unwrap();
   |                          ^^^^^^^^^^^^^^^^^^^ expected i32, found u32
help: you can convert an `u32` to `i32` and panic if the converted value wouldn't fit
   |
33 |     let rlim = getrlimit(libc::RLIMIT_NOFILE.try_into().unwrap()).unwrap();
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[alecmocatta]

Thanks very much for your PRs @dalance! I'm currently away but will review, merge and publish this when I'm back next week.