tiger-vnc: re-adding formula with x11 dep dropped

The formula builds only the client (vncviewer)
which doesn't depend on x11

cf. 
-   http://librelist.com/browser/homebrew/2014/12/31/tiger-vnc-vs-x11/
-   https://github.com/Homebrew/homebrew-x11/pull/12
`require 'formula'` is not needed anymore.