Minor warning about ctype.h function

Building BIND 9.18.19 on NetBSD 10.0_BETA produces this warning:

  CC       libisc_la-httpd.lo
In file included from /usr/include/ctype.h:100,
                 from httpd.c:16:
httpd.c: In function 'value_match':
httpd.c:343:28: warning: array subscript has type 'char' [-Wchar-subscripts]
  343 |   if (isspace(header->value[i])) {
      |                            ^
httpd.c:344:45: warning: array subscript has type 'char' [-Wchar-subscripts]
  344 |    while (i < limit && isspace(header->value[i])) {
      |                                             ^

This is because the valid argument value range is those representable by "unsigned char" and EOF (typically -1). Thus, passing a (signed) char produces undefined behaviour for about half of the value range. Fix? Typically cast argument to "unsigned char".