test_aio_drop doesn't build on OpenBSD

I don't actually know why it doesn't build on OpenBSD, I've added
a conditional compilation attribute for it just to get the test
suite to build.

test/sys/test_aio_drop.rs

@@ -1,6 +1,7 @@
 extern crate nix;
 extern crate tempfile;
 
+#[cfg(not(target_os = "openbsd"))]
 use nix::sys::aio::*;
 use nix::sys::signal::*;
 use std::os::unix::io::AsRawFd;
@@ -11,7 +12,8 @@ use tempfile::tempfile;
 // the AIO subsystem and causes subsequent tests to fail
 #[test]
 #[should_panic(expected = "Dropped an in-progress AioCb")]
-#[cfg(not(target_env = "musl"))]
+#[cfg(not(any(target_env = "musl",
+              target_os = "openbsd")))]
 fn test_drop() {
     const WBUF: &[u8] = b"CDEF";