WinRT: build fix in joystick code

src/joystick/windows/SDL_windowsjoystick_c.h

@@ -83,7 +83,9 @@ struct joystick_hwdata
     DWORD dwPacketNumber;
 };
 
+#if SDL_JOYSTICK_DINPUT
 extern const DIDATAFORMAT SDL_c_dfDIJoystick2;
+#endif
 
 extern void SDL_SYS_AddJoystickDevice(JoyStick_DeviceData *device);