Add guards for mbedtls_psa_crypto_free()

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/programs/pkey/rsa_sign_pss.c b/programs/pkey/rsa_sign_pss.c
index 16dca8b..999669e 100644
--- a/programs/pkey/rsa_sign_pss.c
+++ b/programs/pkey/rsa_sign_pss.c
@@ -162,7 +162,9 @@
     mbedtls_pk_free(&pk);
     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);
 }