gnutls: fix compilation with 3.3.0+

(cherry picked from commit b5a7662a2c735ecb22b23f60a91c4f9c9162f3b8)

modules/misc/gnutls.c

@@ -96,7 +96,7 @@ static int gnutls_Init (vlc_object_t *obj)
         msg_Err (obj, "unsupported GnuTLS version");
         return -1;
     }
-    msg_Dbg (p_this, "using GnuTLS verson %s", version);
+    msg_Dbg (obj, "using GnuTLS verson %s", version);
     return 0;
 }