Revert "asn1: Remove dependency on X.509"

This reverts commit d8087713aea2bf3d61bb2470a8d74409e74907fb.
diff --git a/include/mbedtls/asn1.h b/include/mbedtls/asn1.h
index 33b3004..4c61b6e 100644
--- a/include/mbedtls/asn1.h
+++ b/include/mbedtls/asn1.h
@@ -66,7 +66,7 @@
  * - 0x02 -- tag indicating INTEGER
  * - 0x01 -- length in octets
  * - 0x05 -- value
- * Such sequences are typically read into Mbed TLS's \c mbedtls_x509_buf.
+ * Such sequences are typically read into \c ::mbedtls_x509_buf.
  * \{
  */
 #define MBEDTLS_ASN1_BOOLEAN                 0x01