UWP: Fix VS 2017 build with new `get_unique_id` method

Fixes this error:
```
platform\uwp\os_uwp.cpp(715): error C3149: 'Windows::Storage::Streams::IBuffer': cannot use this type here without a top-level '^'
```

platform/uwp/os_uwp.cpp

@@ -712,7 +712,7 @@ void OS_UWP::set_icon(const Ref<Image> &p_icon) {
 String OS_UWP::get_unique_id() const {
 	// Get the hardware token and read it into an array of bytes.
 	HardwareToken ^ token = HardwareIdentification::GetPackageSpecificToken(nullptr);
-	IBuffer hwId = token->Id;
+	IBuffer ^ hwId = token->Id;
 	DataReader ^ dr = DataReader::FromBuffer(hwId);
 	uint8_t *data = new uint8_t[hwId->Length];
 	dr->ReadBytes(Platform::ArrayReference<uint8_t>(data, hwId->Length));