parse-options: fix declaration of parse_opt_unknown_cb()

In f41179f (parse-options: avoid magic return codes, 2019-01-27),
this function's signature was changed to return an `enum` instead of a
non-descriptive `int`. Which is a good change. However, it forgot to
adjust the declaration. Which we now fix.

This was noticed by the MSVC compiler.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

parse-options.h

@@ -287,7 +287,9 @@ int parse_opt_commit(const struct option *, const char *, int);
 int parse_opt_tertiary(const struct option *, const char *, int);
 int parse_opt_string_list(const struct option *, const char *, int);
 int parse_opt_noop_cb(const struct option *, const char *, int);
-int parse_opt_unknown_cb(struct parse_opt_ctx_t *ctx, const struct option *, const char *, int);
+enum parse_opt_result parse_opt_unknown_cb(struct parse_opt_ctx_t *ctx,
+					   const struct option *opt,
+					   const char *arg, int unset);
 int parse_opt_passthru(const struct option *, const char *, int);
 int parse_opt_passthru_argv(const struct option *, const char *, int);