GnuTLS CURLINFO_TLS_SESSION / CURLINFO_TLS_SSL_PTR type confusion

Summary: `curl_easy_getinfo` for `CURLINFO_TLS_SESSION` and `CURLINFO_TLS_SSL_PTR` incorrectly return `CURLSSLBACKEND_OPENSSL` in `struct curl_tlssessioninfo` `backend` field for GnuTLS.

struct curl_tlssessioninfo {
  curl_sslbackend backend;
  void *internals;
};