Add unused field to mbedtls_x509_crt structure

This field reserves a small space for us to repurpose in the lifetime
of the 4.1 LTS release without breaking the ABI, if needed.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

include/mbedtls/x509_crt.h

diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 8ee7c46..0a7b532 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -82,6 +82,12 @@
     mbedtls_md_type_t MBEDTLS_PRIVATE(sig_md);           /**< Internal representation of the MD algorithm of the signature algorithm, e.g. MBEDTLS_MD_SHA256 */
     mbedtls_pk_sigalg_t MBEDTLS_PRIVATE(sig_pk);           /**< Internal representation of the Public Key algorithm of the signature algorithm, e.g. MBEDTLS_PK_RSA */
 
+    /* Unused field reserved for future use */
+    union {
+        size_t number;
+        void *ptr;
+    } MBEDTLS_PRIVATE(unused);
+
     /** Next certificate in the linked list that constitutes the CA chain.
      * \p NULL indicates the end of the list.
      * Do not modify this field directly. */