lib/cmdline: Export next_arg() for being used in modules

At least one module will benefit from using next_arg() helper.
Let's export it for that module and others if they consider it
helpful.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>