doxygen: add @file declarations for src/core/crypto

If a file doesn't use the file command (either \file or @file),
Doxygen won't try to process it.

src/core/crypto/hs_ntor.h

 /* Copyright (c) 2017-2019, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+/**
+ * @file hs_ntor.h
+ * @brief Header for hs_ntor.c
+ **/
+
 #ifndef TOR_HS_NTOR_H
 #define TOR_HS_NTOR_H
 

src/core/crypto/onion_ntor.h

 /* Copyright (c) 2012-2019, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+/**
+ * @file onion_ntor.h
+ * @brief Header for onion_ntor.c
+ **/
+
 #ifndef TOR_ONION_NTOR_H
 #define TOR_ONION_NTOR_H
 

src/core/crypto/relay_crypto.c

@@ -4,6 +4,11 @@
  * Copyright (c) 2007-2019, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+/**
+ * @file relay_crypto.h
+ * @brief Header for relay_crypto.c
+ **/
+
 #include "core/or/or.h"
 #include "core/or/circuitlist.h"
 #include "core/or/crypt_path.h"