tools/crypto_bench: update the comment about build command

commit cd62f9d557 missing the comment about build

Reviewed-by: Nicolas George <nicolas.george@normalesup.org>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

tools/crypto_bench.c

@@ -19,7 +19,7 @@
  */
 
 /* Optional external libraries; can be enabled using:
- * make VERSUS=crypto+gcrypt+tomcrypt tools/crypto_bench */
+ * make VERSUS=crypto+gcrypt+tomcrypt+mbedcrypto tools/crypto_bench */
 #define USE_crypto           0x01    /* OpenSSL's libcrypto */
 #define USE_gcrypt           0x02    /* GnuTLS's libgcrypt */
 #define USE_tomcrypt         0x04    /* LibTomCrypt */