Port web_service from Citra

src/web_service/json.h

@@ -0,0 +1,18 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+// This hack is needed to support json.hpp on platforms where the C++17 stdlib
+// lacks std::string_view. See https://github.com/nlohmann/json/issues/735.
+// clang-format off
+#if !__has_include(<string_view>) && __has_include(<experimental/string_view>)
+# include <experimental/string_view>
+# define string_view experimental::string_view
+# include <json.hpp>
+# undef string_view
+#else
+# include <json.hpp>
+#endif
+// clang-format on