fix compiler warning

lib/hostip.c

@@ -181,21 +181,21 @@ int Curl_num_addresses(const Curl_addrinfo *addr)
 const char *
 Curl_printable_address(const Curl_addrinfo *ai, char *buf, size_t bufsize)
 {
-  struct sockaddr_in *sa4;
-  struct in_addr *ipaddr4;
+  const struct sockaddr_in *sa4;
+  const struct in_addr *ipaddr4;
 #ifdef ENABLE_IPV6
-  struct sockaddr_in6 *sa6;
-  struct in6_addr *ipaddr6;
+  const struct sockaddr_in6 *sa6;
+  const struct in6_addr *ipaddr6;
 #endif
 
   switch (ai->ai_family) {
     case AF_INET:
-      sa4 = (struct sockaddr_in *)ai->ai_addr;
+      sa4 = (const void *)ai->ai_addr;
       ipaddr4 = &sa4->sin_addr;
       return Curl_inet_ntop(ai->ai_family, (const void *)ipaddr4, buf, bufsize);
 #ifdef ENABLE_IPV6
     case AF_INET6:
-      sa6 = (struct sockaddr_in6 *)ai->ai_addr;
+      sa6 = (const void *)ai->ai_addr;
       ipaddr6 = &sa6->sin6_addr;
       return Curl_inet_ntop(ai->ai_family, (const void *)ipaddr6, buf, bufsize);
 #endif