winealsa: Use a pthread mutex for the stream lock.

dlls/winealsa.drv/unixlib.h

@@ -51,6 +51,8 @@ struct alsa_stream
     BYTE *local_buffer, *tmp_buffer, *remapping_buf, *silence_buf;
     LONG32 getbuf_last; /* <0 when using tmp_buffer */
     float *vols;
+
+    pthread_mutex_t lock;
 };
 
 struct endpoint