make the libidn pointer in the version struct a const

include/curl/curl.h

@@ -1246,7 +1246,7 @@ typedef struct {
   int ares_num;
 
   /* This field was aded in CURLVERSION_THIRD */
-  char *libidn;
+  const char *libidn;
 } curl_version_info_data;
 
 #define CURL_VERSION_IPV6      (1<<0)  /* IPv6-enabled */