Specify xsel and/or xclip in gomuks dependencies

[wvffle]

https://github.com/zyedidia/clipboard specifies that xsel or xclip is required to fetch the clipboard contents. It should be also specified in gomuks dependencies.

[TigerP]

I would say it is a recommendation, because you don't need xsel or xclip to use gomuks. For example on a TTY there is no X, but you can still use gomuks.