Incorrect data32/data64 type in clap_audio_buffer

[ilmai]

In audio-buffer.h, data32 and data64 have types float** and double**, respectively. In clap-sys, these translate to *const *const f32 and *const *const f64. They should be *mut *mut instead to match the C API.

[micahrj]

You're right, and it would be good to fix this going forward. I'm not sure why the pointers were *const in the first place.

It is worth pointing out that this makes no difference in terms of either FFI correctness or Rust safety, so there's no risk of this causing issues for existing code and the fix would be purely for clarity's sake.