gitadora: (arena model) asio option (#718)

## Link to GitHub Issue or related Pull Request, if one exists
#717 

## Description of change
Add an option to override the ASIO device.

By default the game will try to use any ASIO driver that contains the
string `XONAR` in it.

## Testing
Tested with FlexASIO.

src/spice2x/games/gitadora/gitadora.h

@@ -16,6 +16,7 @@ namespace games::gitadora {
     extern std::optional<std::string> SUBSCREEN_OVERLAY_SIZE;
     extern std::optional<socd::SocdAlgorithm> PICK_ALGO;
     extern std::optional<uint8_t> ARENA_WINDOW_COUNT;
+    extern std::optional<std::string> ASIO_DRIVER;
 
     class GitaDoraGame : public games::Game {
     public: