Fix "warning: attribute should be applied to an `extern` block"

[mmastrac]

See message in #70

[mmastrac]

warning: attribute should be applied to an `extern` block
  --> src/lib.rs:24:1
   |
24 |   #[link(name = "c")]
   |   ^^^^^^^^^^^^^^^^^^^
25 | / mod c {
26 | |     extern "C" {}
27 | | }
   | |_- not an `extern` block
   |
   = note: `#[warn(unused_attributes)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!