Fix compilation error with NVCC (#3234)

test/cuda_example/json_cuda.cu

@@ -4,4 +4,8 @@ int main()
 {
     nlohmann::ordered_json json = {"Test"};
     json.dump();
+
+    // regression for #3013 (ordered_json::reset() compile error with nvcc)
+    nlohmann::ordered_json metadata;
+    metadata.erase("key");
 }