pkcs7: Add dependecy on MBEDTLS_MD_C

Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index dcb6392..e5f8b89 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -992,7 +992,8 @@
 #if defined(MBEDTLS_PKCS7_C) && ( ( !defined(MBEDTLS_ASN1_PARSE_C) ) || \
     ( !defined(MBEDTLS_OID_C) ) || ( !defined(MBEDTLS_PK_PARSE_C) ) || \
     ( !defined(MBEDTLS_X509_CRT_PARSE_C) ) ||\
-    ( !defined(MBEDTLS_X509_CRL_PARSE_C) ) || ( !defined(MBEDTLS_BIGNUM_C) ) )
+    ( !defined(MBEDTLS_X509_CRL_PARSE_C) ) || ( !defined(MBEDTLS_BIGNUM_C) ) \
+    ( !defined(MBEDTLS_MD_C) ) )
 #error  "MBEDTLS_PKCS7_C is defined, but not all prerequisites"
 #endif