slirp: fix compilation errors with DEBUG set

slirp: fix compilation errors with DEBUG set

slirp/slirp.c: In function 'get_dns_addr_resolv_conf':
slirp/slirp.c:202:29: error: initialization discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
                 char *res = inet_ntop(af, tmp_addr, s, sizeof(s));
                             ^~~~~~~~~
slirp/slirp.c:204:25: error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
                     res = "(string conversion error)";

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>