revert z_class signedness to suppress compile warning

please, revert z_class to suppress compiler warning
> clang -DHAVE_CONFIG_H -I. -I..      -O2 -pipe -fno-strict-aliasing -MT dns_config.o -MD -MP -MF .deps/dns_config.Tpo -c -o dns_config.o dns_config.c
> dns_config.c:383:69: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
>         if ((zone->z_class = dns_proto_parse_class(tok.tok_string)) < 0) {
>             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
> 1 warning generated.