Please add FreeBSD support

Here https://github.com/RustAudio/baseview/blob/master/src/window.rs#L12 FreeBSD support should be added.

```
@@ -11,6 +11,8 @@
 use crate::win as platform;
 #[cfg(target_os = "linux")]
 use crate::x11 as platform;
+#[cfg(target_os = "freebsd")]
+use crate::x11 as platform;
 
 pub trait WindowHandler {
     fn on_frame(&mut self);
```