Fill in serial field for all HID controllers using hidapi

Controllers/SteelSeriesController/SteelSeriesRivalController.cpp

@@ -54,6 +54,17 @@ char* SteelSeriesRivalController::GetDeviceName()
     return device_name;
 }
 
+std::string SteelSeriesRivalController::GetSerialString()
+{
+    wchar_t serial_string[128];
+    hid_get_serial_number_string(dev, serial_string, 128);
+
+    std::wstring return_wstring = serial_string;
+    std::string return_string(return_wstring.begin(), return_wstring.end());
+
+    return(return_string);
+}
+
 steelseries_type SteelSeriesRivalController::GetMouseType()
 {
     return proto;