Add guards for mbedtls_psa_crypto_free()

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/programs/x509/req_app.c b/programs/x509/req_app.c
index f5fbc79..64b9f0b 100644
--- a/programs/x509/req_app.c
+++ b/programs/x509/req_app.c
@@ -134,7 +134,9 @@
 
 exit:
     mbedtls_x509_csr_free(&csr);
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
     mbedtls_psa_crypto_free();
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
 
     mbedtls_exit(exit_code);
 }