install _grc to /usr/share/zsh/site-functions, fixes #2

This manually installs _grc to zsh's site-functions dir, because install.sh only adds it to $pkgdir with the following condition at end of script.

# probably we should not install it into site-functions in a debian package...
if [ "$PREFIX" = "/usr/local" ]; then
  mkdir -p $PREFIX/zsh/site-functions
  cp -fv _grc $PREFIX/zsh/site-functions/_grc
fi