Fix "array subscript is of type 'char'" on NetBSD 9

Fix "array subscript is of type 'char'" on NetBSD 9

    In file included from rdata.c:602:
    In file included from ./code.h:88:
    ./rdata/in_1/svcb_64.c:259:9: warning: array subscript is of type 'char' [-Wchar-subscripts]
                            if (!isdigit(*region->base)) {
                                 ^~~~~~~~~~~~~~~~~~~~~~
    /usr/include/sys/ctype_inline.h:51:44: note: expanded from macro 'isdigit'
    #define isdigit(c)      ((int)((_ctype_tab_ + 1)[(c)] & _CTYPE_D))
                                                    ^~~~

(cherry picked from commit d09447287f02cdf479cf2e542e4ab0efe7a024fe)