bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Eli Zaretskii]

> Cc: p.stephani2@gmail.com, 32189@debbugs.gnu.org
> From: Ken Brown <kbrown@cornell.edu>
> Date: Fri, 20 Jul 2018 09:49:39 -0400
> 
> ../../master/src/w32cygwinx.c: In function ‘format_string’:
> ../../master/src/w32cygwinx.c:32:3: warning: function ‘format_string’ 
> might be a candidate for ‘gnu_printf’ format attribute 
> [-Wsuggest-attribute=format]
>     Lisp_Object str = vformat_string (format, args);
>     ^~~~~~~~~~~

We do this with vformat_string:

  extern Lisp_Object vformat_string (const char *, va_list)
    ATTRIBUTE_FORMAT_PRINTF (1, 0);

So I guess this new function needs a similar treatment.