fix: add missing deprecation guards

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index f552345..810873c 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -987,7 +987,7 @@
  */
 void mbedtls_x509write_crt_set_version(mbedtls_x509write_cert *ctx, int version);
 
-#if defined(MBEDTLS_BIGNUM_C)
+#if defined(MBEDTLS_BIGNUM_C) && !defined(MBEDTLS_DEPRECATED_REMOVED)
 /**
  * \brief           Set the serial number for a Certificate.
  *
@@ -1005,8 +1005,9 @@
  *
  * \return          0 if successful
  */
-int mbedtls_x509write_crt_set_serial(mbedtls_x509write_cert *ctx, const mbedtls_mpi *serial);
-#endif
+int MBEDTLS_DEPRECATED mbedtls_x509write_crt_set_serial(
+    mbedtls_x509write_cert *ctx, const mbedtls_mpi *serial);
+#endif // MBEDTLS_BIGNUM_C && !MBEDTLS_DEPRECATED_REMOVED
 
 /**
  * \brief           Set the serial number for a Certificate.