remove android build warns

Signed-off-by: fufesou <shuanglongchen@yeah.net>

src/server/audio_service.rs

@@ -64,6 +64,7 @@ mod pa_impl {
                 )))
                 .await
         );
+        #[cfg(target_os = "linux")]
         let zero_audio_frame: Vec<f32> = vec![0.; AUDIO_DATA_SIZE_U8 / 4];
         while sp.ok() && !RESTARTING.load(Ordering::SeqCst) {
             sp.snapshot(|sps| {