Add explicit dependency on libfuzzer-sys

[alexcrichton]

This fixes a build issue with the most recent version of libfuzzer-sys where the prior version of the fuzz_target! macro didn't need the crate as an explicit dependency but the latest version does.

[alexcrichton]

The fuzz build also currently prints out:

warning: for loop over a `Result`. This is more readably written as an `if let` statement
  --> fuzz_targets/moves.rs:65:18
   |
65 |         for i in u.int_in_range(0..=2) {
   |                  ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
   |
65 |         while let Ok(i) = u.int_in_range(0..=2) {
   |         ~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
   |
65 |         if let Ok(i) = u.int_in_range(0..=2) {
   |         ~~~~~~~~~~ ~~~

which is just a warning so not fatal, but I wasn't sure what the intended code here was so I figured I'd ask.

[cfallin]

Thanks!

I'm taking a look at the warning and will address in another PR -- it seems pretty clearly was meant to be for i in 0..limit and the iterability of Result masked the lack of range (yikes!).

[jameysharp]

What libfuzzer-sys change caused this to be necessary? I'd expect this to be a bug in that crate, not something that should be worked around here. I think libfuzzer-sys 0.4.3 should have made this unnecessary, so I'd like to know what broke after that.

In #69 I removed this dependency because it requires keeping dependencies in sync across multiple Cargo.toml files. So I'd like to revert this PR but clearly we need something.

[alexcrichton]

I'm not sure what the change was, I mostly just wanted to fix the fuzz build, I didn't investigate much futher.