autossh: build on darwin too

Tested it out just now - seems like it works fine.

pkgs/tools/networking/autossh/default.nix

···
   meta = with stdenv.lib; {
     homepage = http://www.harding.motd.ca/autossh/;
     description = "Automatically restart SSH sessions and tunnels";
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ pSub ];
   };
 }