Fix the android build

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "cpal"
-version = "0.2.9"
+version = "0.2.10"
 authors = ["Pierre Krieger <pierre.krieger1708@gmail.com>"]
 description = "Cross-platform audio playing library in pure Rust."
 repository = "https://github.com/tomaka/cpal"

src/lib.rs

@@ -43,7 +43,7 @@ extern crate libc;
 
 pub use samples_formats::{SampleFormat, Sample};
 
-#[cfg(all(not(windows), not(unix)))]
+#[cfg(all(not(windows), not(target_os = "linux"), not(target_os = "macos")))]
 use null as cpal_impl;
 
 use std::fmt;