Add guards for mbedtls_psa_crypto_free()

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index e3bd69a..51b09f3 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -894,7 +894,9 @@
     mbedtls_pk_free(&loaded_issuer_key);
     mbedtls_ctr_drbg_free(&ctr_drbg);
     mbedtls_entropy_free(&entropy);
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
     mbedtls_psa_crypto_free();
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
 
     mbedtls_exit(exit_code);
 }