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 63bc755..effff25 100644
--- a/programs/pkey/rsa_sign_pss.c
+++ b/programs/pkey/rsa_sign_pss.c
@@ -158,7 +158,9 @@
     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 */
 
 #if defined(_WIN32)
     mbedtls_printf("  + Press Enter to exit this program.\n");