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>

lib/cmdline.c

@@ -272,3 +272,4 @@ char *next_arg(char *args, char **param, char **val)
 	/* Chew up trailing spaces. */
 	return skip_spaces(args);
 }
+EXPORT_SYMBOL(next_arg);