Fix compilation on ARM (for Rust 1.5)

[MagaTailor]

No description provided.

[hannobraun]

I currently have no way to test on ARM, but I trust that this is correct.

Thanks a lot!

[MagaTailor]

No worries, I've been tracking (and fixing from time to time) this issue since October.

Otherwise, you'd get this error:

Compiling inotify v0.2.2 (file:///tmp/inotify-rs-master)
src/wrapper.rs:60:17: 60:31 error: mismatched types:
 expected `*const u8`,
    found `*const i8`
(expected u8,
    found i8) [E0308]
src/wrapper.rs:60                 c_str.as_ptr(),
                                  ^~~~~~~~~~~~~~
src/wrapper.rs:60:17: 60:31 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error
Could not compile `inotify`.

[hannobraun]

I'm not that worried, or I wouldn't have merged :)

Thanks for the explanation though!