Fix MISRA rule 8.4 Part 4

Rule 8.4: A compatible declaration shall be visible when
          an object or function with external linkage is defined

Fixed for:
	make DEBUG=1 PLAT=fvp SPD=tspd TRUSTED_BOARD_BOOT=1 \
	     GENERATE_COT=1 ARM_ROTPK_LOCATION=devel_rsa \
	     ROT_KEY=arm_rotprivk_rsa.pem MBEDTLS_DIR=mbedtls all

Change-Id: Ie4cd6011b3e4fdcdd94ccb97a7e941f3b5b7aeb8
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/drivers/auth/crypto_mod.c b/drivers/auth/crypto_mod.c
index 3b3e3ac..4cd0550 100644
--- a/drivers/auth/crypto_mod.c
+++ b/drivers/auth/crypto_mod.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -9,7 +9,6 @@
 #include <debug.h>
 
 /* Variable exported by the crypto library through REGISTER_CRYPTO_LIB() */
-extern const crypto_lib_desc_t crypto_lib_desc;
 
 /*
  * The crypto module is responsible for verifying digital signatures and hashes.