xtensa message pluralization

[augustine.sterling@gmail.com]

On Tue, Nov 7, 2017 at 12:27 AM, Alan Modra <amodra@gmail.com> wrote:
> I'm not too certain whether this is needed, ie. whether any of the slot
> and opcode counts can be one.  Please review.
>
>         * config/tc-xtensa.c (finish_vinsn): Properly pluralize error message.

Thanks for the fix. Approved.