parseopt: move definition of enum parse_opt_result up

parseopt: move definition of enum parse_opt_result up

Define enum parse_opt_result before using it in a typedef.  This avoids
the following compiler warning:

   ./parse-options.h:53:14: error: ISO C forbids forward references to 'enum' types [-Werror,-Wpedantic]
   typedef enum parse_opt_result parse_opt_ll_cb(struct parse_opt_ctx_t *ctx,
                ^

While GCC and Clang both accept such a forward reference by default,
other compilers might be less forgiving.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>