pkcs7: Add dependecy on MBEDTLS_MD_C

Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 45dd274..84dcf47 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -2669,7 +2669,8 @@
  * Module:  library/pkcs7.c
  *
  * Requires: MBEDTLS_ASN1_PARSE_C, MBEDTLS_OID_C, MBEDTLS_PK_PARSE_C,
- *           MBEDTLS_X509_CRT_PARSE_C MBEDTLS_X509_CRL_PARSE_C, MBEDTLS_BIGNUM_C
+ *           MBEDTLS_X509_CRT_PARSE_C MBEDTLS_X509_CRL_PARSE_C,
+ *           MBEDTLS_BIGNUM_C, MBEDTLS_MD_C
  *
  * This module is required for the PKCS7 parsing modules.
  */