luajit: point /usr/bin/lua to luajit if lua not selected

The luajit package is a provider of a lua interpreter, but does not install a
lua executable.  This is fine for scripts that explicitly invoke luajit, but
not so good for scripts that just need a lua interpreter and call lua.  This
mod creates a lua symlink so that the non-jit-specific scripts will still work.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>