bash: use -std=gnu17 also for native CFLAGS

bash: use -std=gnu17 also for native CFLAGS

* fixes builds on host with gcc-15:
  http://errors.yoctoproject.org/Errors/Details/853016/

../../bash-5.2.37/builtins/mkbuiltins.c:268:29: error: too many arguments to function ‘xmalloc’; expected 0, have 1
  268 |           error_directory = xmalloc (2 + strlen (argv[arg_index]));
      |                             ^~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>