Link against to fix ABI compat with FreeBSD12.
struct kevent was modified in FreeBSD12.  The symbol supports the old
structure ABI still.
This allows the `mio` crate tests to now pass on FreeBSD12.
@@ -1052,6 +1052,7 @@ extern {
serv: *mut ::c_char,
servlen: ::size_t,
flags: ::c_int) -> ::c_int;
+ #[cfg_attr(target_os = "freebsd", link_name = "kevent@FBSD_1.0")]
pub fn kevent(kq: ::c_int,
changelist: *const ::kevent,
nchanges: ::c_int,
