gallium/tests: fix unused-but-set-variable warning

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7713>

src/gallium/tests/unit/u_cache_test.c

@@ -89,6 +89,7 @@ int main() {
          *key = 0xdeadbeef;
          value_out = (cache_test_value *) util_cache_get(cache, key);
          assert(value_out == NULL);
+         (void)value_out;
          free(key);