Don't turn C-u into a prefix command

* elisp/ghc.el
  * There are many ways to fix issue #351 so this is
    just one example and may not fit the style you
    prefer, so please feel free to use or not.
    It is easy to work-around the issue by setting
    ghc-initial-sig-key to another key combination
    before ghc-init, but it would be nice to have a
    fix. The use of C-u for this binding seems to
    be just as a flag, thus this change takes
    the simplest fix to achieve that purpose.