cert_[req/write]: add MD_C dependency

Both programs use mbedtls_md_info_from_string() which is only
available as long as MBEDTLS_MD_C is enabled.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index bf25c4c..0b2575e 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -15,7 +15,7 @@
     !defined(MBEDTLS_X509_CRT_PARSE_C) || !defined(MBEDTLS_FS_IO) || \
     !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C) || \
     !defined(MBEDTLS_ERROR_C) || !defined(MBEDTLS_MD_CAN_SHA256) || \
-    !defined(MBEDTLS_PEM_WRITE_C)
+    !defined(MBEDTLS_PEM_WRITE_C) || !defined(MBEDTLS_MD_C)
 int main(void)
 {
     mbedtls_printf("MBEDTLS_X509_CRT_WRITE_C and/or MBEDTLS_X509_CRT_PARSE_C and/or "