util/guest-random: Clean up global variable shadowing

util/guest-random: Clean up global variable shadowing

Fix:

  util/guest-random.c:90:45: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  int qemu_guest_random_seed_main(const char *optarg, Error **errp)
                                              ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/getopt.h:77:14: note: previous declaration is here
  extern char *optarg;                    /* getopt(3) external variables */
               ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004120019.93101-13-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>