Fix fd_set for DragonFly

Author: mneumann

src/unix/bsd/mod.rs

@@ -68,10 +68,10 @@ s! {
 
     pub struct fd_set {
         #[cfg(all(target_pointer_width = "64",
-                  target_os = "freebsd"))]
+                  any(target_os = "freebsd", target_os = "dragonfly")))]
         fds_bits: [i64; FD_SETSIZE / 64],
         #[cfg(not(all(target_pointer_width = "64",
-                      target_os = "freebsd")))]
+                      any(target_os = "freebsd", target_os = "dragonfly"))))]
         fds_bits: [i32; FD_SETSIZE / 32],
     }