Pointers should use usize or isize, instead of i64

libspa fails to build without some type coercion on 32-bit platforms (such as i686) because some places in both libspa-sys and libspa expects pointers to be i64 instead of usize or isize

libc seems to use usize as the counterpart of glibc's size_t, so maybe that is what should be used?

https://docs.rs/libc/latest/libc/type.size_t.html

If there is a reason to prefer signed integers then maybe isize can be used