Fixed building on Windows with SDL_VIDEO=OFF

Fixes https://github.com/libsdl-org/SDL/issues/6562

src/joystick/windows/SDL_dinputjoystick.c

@@ -42,7 +42,11 @@
 #define CONVERT_MAGNITUDE(x)    (((x)*10000) / 0x7FFF)
 
 /* external variables referenced. */
+#ifdef SDL_VIDEO_DRIVER_WINDOWS
 extern HWND SDL_HelperWindow;
+#else
+static const HWND SDL_HelperWindow = NULL;
+#endif
 
 /* local variables */
 static SDL_bool coinitialized = SDL_FALSE;