gitadora: (arena model) prevent the small subscreen from being closed (#487)

## Link to GitHub Issue, if one exists
#477 

## Description of change
Ensure the small subscreen can't be resized or closed by accident.

src/spice2x/hooks/graphics/graphics.h

@@ -60,6 +60,7 @@ extern bool GRAPHICS_WSUB_BORDERLESS;
 extern bool GRAPHICS_WSUB_ALWAYS_ON_TOP;
 extern HWND TDJ_SUBSCREEN_WINDOW;
 extern HWND SDVX_SUBSCREEN_WINDOW;
+extern HWND GFDM_SUBSCREEN_WINDOW;
 
 extern bool SUBSCREEN_FORCE_REDRAW;