src/c-types-parse.lisp: Handle `...' in prefix-`(' disambiguation.

src/c-types-parse.lisp: Handle `...' in prefix-`(' disambiguation.

Now that we've actually got the machinery to parse ellipses in function
argument lists, it'd be nice to do it properly.

(I think -- but I might be wrong -- that the reason this wasn't done
before is that C doesn't let you put `...' as the only thing in an
argument list, because you need to have a named argument to initialize
the `va_list'.  But Sod can, and does, because there's always at least a
receiver argument.)