added comment on a variable that is unused on some platforms

lib/hostip.c

@@ -111,7 +111,7 @@ struct hostent *GetHost(struct UrlData *data,
 {
   struct hostent *h = NULL;
   unsigned long in;
-  int ret;
+  int ret; /* this variable is unused on several platforms but used on some */
 
 #define CURL_NAMELOOKUP_SIZE 9000