Revert "md: Remove dependency on X.509"

This reverts commit ebbc5f7940e5271d3cdd31818119d558ba040155.
diff --git a/library/md.c b/library/md.c
index e235bc8..b56c2dd 100644
--- a/library/md.c
+++ b/library/md.c
@@ -138,7 +138,7 @@
 #endif
 
 /*
- * Reminder: update profiles in Mbed TLS's x509_crt.c when adding a new hash!
+ * Reminder: update profiles in x509_crt.c when adding a new hash!
  */
 static const int supported_digests[] = {