Fix lifetimes for AudioSpec.

The new rustc seems to complain about missing lifetime parameters on `AudioSpec`.

    src/sdl2/audio.rs:264:100: 264:109 error: wrong number of lifetime parameters: expected 1 but found 0 [E0107]
    src/sdl2/audio.rs:264     pub fn open(device: Option<&str>, iscapture: int, spec: &AudioSpec) -> SdlResult<(AudioDevice, AudioSpec)> {

We seem to nee the extra `'call` lifetime parameter on `AudioSpec::load_wav_rw` because
rustc seems to infer `'a` otherwise.