Raise GnuTLS requirements to 3.x

This allows us to simplify things by getting rid of some old
compatibility code. People should really be using current versions
of GnuTLS anyway to stay secure.
diff --git a/common/rfb/CSecurityTLS.h b/common/rfb/CSecurityTLS.h
index f5f10e4..b147d80 100644
--- a/common/rfb/CSecurityTLS.h
+++ b/common/rfb/CSecurityTLS.h
@@ -64,9 +64,9 @@
   private:
     static void initGlobal();
 
-    gnutls_session session;
-    gnutls_anon_client_credentials anon_cred;
-    gnutls_certificate_credentials cert_cred;
+    gnutls_session_t session;
+    gnutls_anon_client_credentials_t anon_cred;
+    gnutls_certificate_credentials_t cert_cred;
     bool anon;
 
     char *cafile, *crlfile;