Add guards for mbedtls_psa_crypto_free()

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/programs/ssl/dtls_server.c b/programs/ssl/dtls_server.c
index 568ef1f..2128d02 100644
--- a/programs/ssl/dtls_server.c
+++ b/programs/ssl/dtls_server.c
@@ -404,7 +404,9 @@
 #endif
     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)
     printf("  Press Enter to exit this program.\n");